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# Source code for torch.utils.checkpoint

    import contextlib
    import uuid
    import warnings
    import weakref
    from collections import defaultdict
    from typing import (
        Any,
        Callable,
        ContextManager,
        DefaultDict,
        Dict,
        Iterable,
        List,
        Optional,
        Tuple,
    )
    from weakref import ReferenceType
    from torch.testing._internal.logging_tensor import LoggingTensorMode, capture_logs
    import platform

    import torch

    __all__ = [
        "checkpoint",
        "checkpoint_sequential",
        "CheckpointError",
        "CheckpointFunction",
        "check_backward_validity",
        "detach_variable",
        "get_device_states",
        "set_device_states",
        "noop_context_fn",
        "set_checkpoint_early_stop",
        "DefaultDeviceType",
    ]

    _DEFAULT_DETERMINISM_MODE = "default"

    def detach_variable(inputs: Tuple[Any, ...]) -> Tuple[torch.Tensor, ...]:
        if isinstance(inputs, tuple):
            out = []
            for inp in inputs:
                if not isinstance(inp, torch.Tensor):
                    out.append(inp)
                    continue

                x = inp.detach()
                x.requires_grad = inp.requires_grad
                out.append(x)
            return tuple(out)
        else:
            raise RuntimeError(
                "Only tuple of tensors is supported. Got Unsupported input type: ",
                type(inputs).__name__,
            )

    def check_backward_validity(inputs: Iterable[Any]) -> None:
        if not any(inp.requires_grad for inp in inputs if isinstance(inp, torch.Tensor)):
            warnings.warn(
                "None of the inputs have requires_grad=True. Gradients will be None"
            )

    def _get_device_module(device="cuda"):
        device_module = getattr(torch, device)
        return device_module

    class DefaultDeviceType:
        r"""
        A class that manages the default device type for checkpointing.
        If no non-CPU tensors are present, the default device type will
        be used. The default value is 'cuda'. The device type is used in
        the checkpointing process when determining which device states
        to save and restore for recomputation.
        """
        _default_device_type = "cuda"

        @staticmethod
        def set_device_type(device: str = "cuda"):
            """
            Set the default device type for checkpointing.

            Args:
                device (str): The device type to be set as default. Default is 'cuda'.
            """
            DefaultDeviceType._default_device_type = device

        @staticmethod
        def get_device_type() -> str:
            """
            Get the current default device type for checkpointing.

            Returns:
                str: The current default device type.
            """
            return DefaultDeviceType._default_device_type

    def _infer_device_type(*args):
        device_types = list(
            {
                arg.device.type
                for arg in args
                if isinstance(arg, torch.Tensor) and not arg.device.type == "cpu"
            }
        )
        if len(device_types) > 1:
            warnings.warn(
                "Tensor arguments, excluding CPU tensors, are detected on at least two types of devices. "
                "Device state will only be saved for devices of a single device type, and the remaining "
                "devices will be ignored. Consequently, if any checkpointed functions involve randomness, "
                "this may result in incorrect gradients. (Note that if CUDA devices are among the devices "
                "detected, it will be prioritized; otherwise, the first device encountered will be selected.)"
            )
        if len(device_types) == 0:
            return DefaultDeviceType.get_device_type()
        elif "cuda" in device_types:
            return "cuda"
        else:
            return device_types[0]

    # We can't know if the run_fn will internally move some args to different devices,
    # which would require logic to preserve rng states for those devices as well.
    # We could paranoically stash and restore ALL the rng states for all visible devices,
    # but that seems very wasteful for most cases.  Compromise:  Stash the RNG state for
    # the device of all Tensor args.
    #
    # To consider:  maybe get_device_states and set_device_states should reside in torch/random.py?
    def get_device_states(*args) -> Tuple[List[int], List[torch.Tensor]]:
        # This will not error out if "arg" is a CPU tensor or a non-tensor type because
        # the conditionals short-circuit.
        fwd_device_ids = list(
            {
                arg.get_device()
                for arg in args
                if isinstance(arg, torch.Tensor) and not arg.device.type == "cpu"
            }
        )

        fwd_device_states = []
        device_module = _get_device_module(_infer_device_type(*args))

        for device_id in fwd_device_ids:
            with device_module.device(device_id):
                fwd_device_states.append(device_module.get_rng_state())

        return fwd_device_ids, fwd_device_states

    def set_device_states(devices, states) -> None:
        device_module = _get_device_module(_infer_device_type(*states))
        for device, state in zip(devices, states):
            with device_module.device(device):
                device_module.set_rng_state(state)

    def _get_autocast_kwargs(device="cuda"):
        if device == "cuda":
            device_autocast_kwargs = {
                "enabled": torch.is_autocast_enabled(),
                "dtype": torch.get_autocast_gpu_dtype(),
                "cache_enabled": torch.is_autocast_cache_enabled(),
            }
        elif _supports_autocast(device):
            device_module = _get_device_module(device)
            device_autocast_kwargs = {
                "enabled": device_module.is_autocast_enabled(),
                "dtype": device_module.get_autocast_dtype(),
                "cache_enabled": torch.is_autocast_cache_enabled(),
            }
        else:
            device_autocast_kwargs = None

        cpu_autocast_kwargs = {
            "enabled": torch.is_autocast_cpu_enabled(),
            "dtype": torch.get_autocast_cpu_dtype(),
            "cache_enabled": torch.is_autocast_cache_enabled(),
        }

        return device_autocast_kwargs, cpu_autocast_kwargs

    def _supports_autocast(device):
        device_module = _get_device_module(device)
        return device == "cuda" or (hasattr(device_module, "is_autocast_enabled")
                                    and hasattr(device_module, "get_autocast_dtype"))

    class CheckpointFunction(torch.autograd.Function):
        @staticmethod
        def forward(ctx, run_function, preserve_rng_state, *args):
            check_backward_validity(args)
            ctx.run_function = run_function
            ctx.preserve_rng_state = preserve_rng_state
            # Accommodates the (remote) possibility that autocast is enabled for cpu AND gpu.
            ctx.device = _infer_device_type(*args)
            ctx.device_autocast_kwargs, ctx.cpu_autocast_kwargs = _get_autocast_kwargs(
                ctx.device
            )
            if preserve_rng_state:
                ctx.fwd_cpu_state = torch.get_rng_state()
                # Don't eagerly initialize the cuda context by accident.
                # (If the user intends that the context is initialized later, within their
                # run_function, we SHOULD actually stash the cuda state here.  Unfortunately,
                # we have no way to anticipate this will happen before we run the function.)
                ctx.had_device_in_fwd = False
                device_module = _get_device_module(ctx.device)
                if getattr(device_module, "_initialized", False):
                    ctx.had_device_in_fwd = True
                    ctx.fwd_devices, ctx.fwd_device_states = get_device_states(*args)

            # Save non-tensor inputs in ctx, keep a placeholder None for tensors
            # to be filled out during the backward.
            ctx.inputs = []
            ctx.tensor_indices = []
            tensor_inputs = []
            for i, arg in enumerate(args):
                if torch.is_tensor(arg):
                    tensor_inputs.append(arg)
                    ctx.tensor_indices.append(i)
                    ctx.inputs.append(None)
                else:
                    ctx.inputs.append(arg)

            ctx.save_for_backward(*tensor_inputs)

            with torch.no_grad():
                outputs = run_function(*args)
            return outputs

        @staticmethod
        def backward(ctx, *args):
            if not torch.autograd._is_checkpoint_valid():
                raise RuntimeError(
                    "Checkpointing is not compatible with .grad() or when an `inputs` parameter"
                    " is passed to .backward(). Please use .backward() and do not pass its `inputs`"
                    " argument."
                )
            # Copy the list to avoid modifying original list.
            inputs = list(ctx.inputs)
            tensor_indices = ctx.tensor_indices
            tensors = ctx.saved_tensors
            device_module = _get_device_module(ctx.device)

            # Fill in inputs with appropriate saved tensors.
            for i, idx in enumerate(tensor_indices):
                inputs[idx] = tensors[i]

            # Stash the surrounding rng state, and mimic the state that was
            # present at this time during forward.  Restore the surrounding state
            # when we're done.
            rng_devices = []
            if ctx.preserve_rng_state and ctx.had_device_in_fwd:
                rng_devices = ctx.fwd_devices
            with torch.random.fork_rng(
                devices=rng_devices, enabled=ctx.preserve_rng_state, device_type=ctx.device
            ):
                if ctx.preserve_rng_state:
                    torch.set_rng_state(ctx.fwd_cpu_state)
                    if ctx.had_device_in_fwd:
                        set_device_states(ctx.fwd_devices, ctx.fwd_device_states)
                detached_inputs = detach_variable(tuple(inputs))

                device_autocast_ctx = device_module.amp.autocast(
                    **ctx.device_autocast_kwargs
                ) if _supports_autocast(ctx.device) else contextlib.nullcontext()
                with torch.enable_grad(), device_autocast_ctx, \
                     torch.cpu.amp.autocast(**ctx.cpu_autocast_kwargs):
                    outputs = ctx.run_function(*detached_inputs)

            if isinstance(outputs, torch.Tensor):
                outputs = (outputs,)

            # run backward() with only tensor that requires grad
            outputs_with_grad = []
            args_with_grad = []
            for i in range(len(outputs)):
                if torch.is_tensor(outputs[i]) and outputs[i].requires_grad:
                    outputs_with_grad.append(outputs[i])
                    args_with_grad.append(args[i])
            if len(outputs_with_grad) == 0:
                raise RuntimeError(
                    "none of output has requires_grad=True,"
                    " this checkpoint() is not necessary"
                )
            torch.autograd.backward(outputs_with_grad, args_with_grad)
            grads = tuple(
                inp.grad if isinstance(inp, torch.Tensor) else None
                for inp in detached_inputs
            )

            return (None, None) + grads

    def noop_context_fn():
        return contextlib.nullcontext(), contextlib.nullcontext()

    # TorchDynamo does not step inside utils.checkpoint function.  The flow
    # looks likes this
    #  1) TorchDynamo tries to wrap utils.checkpoint in a HigherOrderOp by
    #     speculatively checking if the forward function is safe to trace.
    #  2) If yes, then Dynamo-generated Fx graph has the wrapped higher
    #     order op. As a result, TorchDynamo does not look inside utils.checkpoint.
    #  3) If not, then TorchDynamo falls back to eager by performing a graph
    #     break. And here, the following disable wrapper ensures that
    #     TorchDynamo does not trigger again on the frames created by
    #     utils.checkpoint innards.
    @torch._disable_dynamo
    def checkpoint(
        function,
        *args,
        use_reentrant: Optional[bool] = None,
        context_fn: Callable[[], Tuple[ContextManager, ContextManager]] = noop_context_fn,
        determinism_check: str = _DEFAULT_DETERMINISM_MODE,
        debug: bool = False,
        **kwargs
    ):
        r"""Checkpoint a model or part of the model

        Activation checkpointing is a technique that trades compute for memory.
        Instead of keeping tensors needed for backward alive until they are used in
        gradient computation during backward, forward computation in checkpointed
        regions omits saving tensors for backward and recomputes them during the
        backward pass. Activation checkpointing can be applied to any part of a
        model.

        There are currently two checkpointing implementations available, determined
        by the :attr:`use_reentrant` parameter. It is recommended that you use
        ``use_reentrant=False``. Please refer the note below for a discussion of
        their differences.

        .. warning::

            If the :attr:`function` invocation during the backward pass differs
            from the forward pass, e.g., due to a global variable, the checkpointed
            checkpointed version may not be equivalent, potentially causing an
            error being raised or leading to silently incorrect gradients.

        .. warning::

            If you are using the ``use_reentrant=True`` variant (this is currently
            the default), please refer to the note below for important
            considerations and potential limitations.

        .. note::

            The reentrant variant of checkpoint (``use_reentrant=True``) and
            the non-reentrant variant of checkpoint (``use_reentrant=False``)
            differ in the following ways:

            * Non-reentrant checkpoint stops recomputation as soon as all needed
              intermediate activations have been recomputed. This feature is enabled
              by default, but can be disabled with :func:`set_checkpoint_early_stop`.
              Reentrant checkpoint always recomputes :attr:`function` in its
              entirety during the backward pass.

            * The reentrant variant does not record the autograd graph during the
              forward pass, as it runs with the forward pass under
              :func:`torch.no_grad`. The non-reentrant version does record the
              autograd graph, allowing one to perform backward on the graph within
              checkpointed regions.

            * The reentrant checkpoint only supports the
              :func:`torch.autograd.backward` API for the backward pass without its
              `inputs` argument, while the non-reentrant version supports all ways
              of performing the backward pass.

            * At least one input and output must have ``requires_grad=True`` for the
              reentrant variant. If this condition is unmet, the checkpointed part
              of the model will not have gradients. The non-reentrant version does
              not have this requirement.

            * The reentrant version does not consider tensors in nested structures
              (e.g., custom objects, lists, dicts, etc) as participating in
              autograd, while the non-reentrant version does.

            * The reentrant checkpoint does not support checkpointed regions with
              detached tensors from the computational graph, whereas the
              non-reentrant version does. For the reentrant variant, if the
              checkpointed segment contains tensors detached using ``detach()`` or
              with :func:`torch.no_grad`, the backward pass will raise an error.
              This is because ``checkpoint`` makes all the outputs require gradients
              and this causes issues when a tensor is defined to have no gradient in
              the model. To avoid this, detach the tensors outside of the
              ``checkpoint`` function.

        Args:
            function: describes what to run in the forward pass of the model or
                part of the model. It should also know how to handle the inputs
                passed as the tuple. For example, in LSTM, if user passes
                ``(activation, hidden)``, :attr:`function` should correctly use the
                first input as ``activation`` and the second input as ``hidden``
            preserve_rng_state(bool, optional):  Omit stashing and restoring
                the RNG state during each checkpoint.
                Default: ``True``
            use_reentrant(bool, optional): Use checkpointing
                implementation that requires re-entrant autograd.
                If ``use_reentrant=False`` is specified, ``checkpoint`` will use an
                implementation that does not require re-entrant autograd. This
                allows ``checkpoint`` to support additional functionality, such as
                working as expected with ``torch.autograd.grad`` and support for
                keyword arguments input into the checkpointed function. Note that future
                versions of PyTorch will default to ``use_reentrant=False``.
                Default: ``True``
            context_fn(Callable, optional): A callable returning a tuple of two
                context managers. The function and its recomputation will be run
                under the first and second context managers respectively.
                This argument is only supported if ``use_reentrant=False``.
            determinism_check(str, optional): A string specifying the determinism
                check to perform. By default it is set to ``"default"`` which
                compares the shapes, dtypes, and devices of the recomputed tensors
                against those the saved tensors. To turn off this check, specify
                ``"none"``. Currently these are the only two supported values.
                Please open an issue if you would like to see more determinism
                checks. This argument is only supported if ``use_reentrant=False``,
                if ``use_reentrant=True``, the determinism check is always disabled.
            debug(bool, optional): If ``True``, error messages will also include
                a trace of the operators ran during the original forward computation
                as well as the recomputation. This argument is only supported if
                ``use_reentrant=False``.
            args: tuple containing inputs to the :attr:`function`

        Returns:
            Output of running :attr:`function` on :attr:`*args`
        """
        if use_reentrant is None:
            warnings.warn(
                "torch.utils.checkpoint: please pass in use_reentrant=True or "
                "use_reentrant=False explicitly. The default value of use_reentrant "
                "will be updated to be False in the future. To maintain current "
                "behavior, pass use_reentrant=True. It is recommended that you use "
                "use_reentrant=False. Refer to docs for more details on the "
                "differences between the two variants."
            )
            use_reentrant = True
        # Hack to mix *args with **kwargs in a python 2.7-compliant way
        preserve = kwargs.pop("preserve_rng_state", True)
        if kwargs and use_reentrant:
            raise ValueError(
                "Unexpected keyword arguments: " + ",".join(arg for arg in kwargs)
            )

        if use_reentrant:
            if context_fn is not noop_context_fn or debug is not False:
                raise ValueError(
                    "Passing `context_fn` or `debug` is only supported when "
                    "use_reentrant=False."
                )
            return CheckpointFunction.apply(function, preserve, *args)
        else:
            gen = _checkpoint_without_reentrant_generator(
                function, preserve, context_fn, determinism_check, debug, *args, **kwargs
            )
            # Runs pre-forward logic
            next(gen)
            ret = function(*args, **kwargs)
            # Runs post-forward logic
            try:
                next(gen)
            except StopIteration:
                return ret

    def checkpoint_sequential(functions, segments, input, use_reentrant=True, **kwargs):
        r"""A helper function for checkpointing sequential models.

        Sequential models execute a list of modules/functions in order
        (sequentially). Therefore, we can divide such a model in various segments
        and checkpoint each segment. All segments except the last will not store
        the intermediate  activations. The inputs of each checkpointed segment will
        be saved for re-running the segment in the backward pass.

        .. warning::
            If you are using the ``use_reentrant=True` variant (this is the
            default), please see :func:`~torch.utils.checkpoint.checkpoint` for
            the important considerations and limitations of this variant. It is
            recommended that you use ``use_reentrant=False``.

        .. warning:
            Since PyTorch 1.4, it allows only one Tensor as the input and
            intermediate outputs, just like :class:`torch.nn.Sequential`.

        Args:
            functions: A :class:`torch.nn.Sequential` or the list of modules or
                functions (comprising the model) to run sequentially.
            segments: Number of chunks to create in the model
            input: A Tensor that is input to :attr:`functions`
            preserve_rng_state(bool, optional):  Omit stashing and restoring
                the RNG state during each checkpoint.
                Default: ``True``
            use_reentrant(bool, optional): Use checkpointing
                implementation that requires re-entrant autograd.
                If ``use_reentrant=False`` is specified, ``checkpoint`` will use an
                implementation that does not require re-entrant autograd. This
                allows ``checkpoint`` to support additional functionality, such as
                working as expected with ``torch.autograd.grad`` and support for
                keyword arguments input into the checkpointed function.
                Default: ``True``

        Returns:
            Output of running :attr:`functions` sequentially on :attr:`*inputs`

        Example:
            >>> # xdoctest: +SKIP("stub")
            >>> model = nn.Sequential(...)
            >>> input_var = checkpoint_sequential(model, chunks, input_var)
        """
        # Hack for keyword-only parameter in a python 2.7-compliant way
        preserve = kwargs.pop("preserve_rng_state", True)
        if kwargs:
            raise ValueError(
                "Unexpected keyword arguments: " + ",".join(arg for arg in kwargs)
            )

        def run_function(start, end, functions):
            def forward(input):
                for j in range(start, end + 1):
                    input = functions[j](input)
                return input

            return forward

        if isinstance(functions, torch.nn.Sequential):
            functions = list(functions.children())

        segment_size = len(functions) // segments
        # the last chunk has to be non-volatile
        end = -1
        for start in range(0, segment_size * (segments - 1), segment_size):
            end = start + segment_size - 1
            input = checkpoint(
                run_function(start, end, functions),
                input,
                use_reentrant=use_reentrant,
                preserve_rng_state=preserve,
            )
        return run_function(end + 1, len(functions) - 1, functions)(input)

    def _internal_assert(cond):
        if not cond:
            raise AssertionError(
                "Something went unexpectedly wrong in activation checkpoint. "
                "Please report this bug by filing an issue to PyTorch."
            )

    # NOTE [ Nestable Checkpoint ]
    #
    # The semantics of nested checkpoint can be defined by two basic rules.
    # Following the two rules leads to an important implication that is central
    # to motivating the design.
    #
    # Rule 1. Saved tensors are managed by inner-most checkpoint only and hidden
    #         from any outer layers of checkpoint.
    #
    # Rule 2. The inputs of inner checkpoints are treated as tensors saved to its
    #         parent checkpoint.
    #
    # Implication: To recompute any given saved tensor, we need to recompute all of
    #              the checkpoints wrapping it.
    #
    # Why is this implied? To unpack a saved tensor X during backward we need to
    # recompute the inner-most checkpoint (#1), and in order to recompute that
    # checkpoint I need to have its inputs, which are managed by that checkpoint's
    # parent (#2), which thus also needs to be recomputed first. Continue this line
    # of reasoning and we realize that in order to unpack X, all checkpoints that
    # were active at the time X was saved need to be recomputed. (unless we have
    # already done so in that backward for some other saved tensor).
    #
    # In practice, we use a noop autograd Function to save inputs as saved tensors.
    # During unpack calling ctx.saved_tensor triggers the parent checkpoint to
    # recompute.
    #
    # Rule 3. We should start recomputation as if there are no checkpoints currently
    #         active. Checkpoints encountered during recomputation are still
    #         respected.
    #
    # When we start recomputation, we push the saved variable hook meant for
    # recomputation on the stack. See examples in Rule 6 for more context.
    #
    #                                  * * * *
    #
    # Beyond the basic semantics specific to nested checkpoint, we impose several
    # more constraints that may apply to checkpointing in general.
    #
    # Rule 4. Lifetime of recomputed tensors
    #
    #         Recomputed tensors are considered specific to particular invocations
    #         of backward and are always cleared immediately as they are unpacked
    #         Particularly, we require this to happen even if retain_graph=True.
    #
    # [ Implementation details of Rule 4 ]
    #
    # If we were okay with recomputed tensors staying alive after backward is run
    # with retain_graph=True, we would store recomputed variables as the values of a
    # WeakKeyDictionary and pack strong references to the keys, so that as we
    # backward, those packed keys would be cleared as long as retain_graph=False.
    # Clearing the packed key clears the corresponding entry in the WKD.
    #
    # If we wish recomputed variables to be immediately cleared as we unpack them in
    # the retain_graph=True case, we cannot rely on the packed keys to be cleared by
    # backward automatically. Instead of packing the strong reference to the key
    # directly, we pack a container object, which we manually clear as we unpack.
    #
    # An important detail is that if a second backward happens, the second
    # recomputation needs to reset the container with a newly created key.
    #
    # Rule 5. Stop recomputation as soon as we've recomputed the saved tensors we
    #         know we need.
    #
    # [ Implementation details of Rule 5 ]
    #
    # During recomputation, raise an exception if the number of recomputed tensors
    # matches the number of tensors that we expected to recompute. We wrap the
    # recomputation call with a try-catch to catch this specific exception. See
    # Rule #6 below for some examples.
    #
    # Rule 6. We support doing backward inside checkpoint context
    #
    # [ retain_graph is True]
    #
    # def fn(x):
    #   y = x.sin()
    #   z = y.cos()
    #   gx, = torch.autograd.grad(z, x, retains_grad=True)
    #   return gx, z
    #
    # out = checkpoint(fn)(inp)
    # out.backward()
    #
    # Because z is saved by cos while checkpoint is enabled, it would not be
    # actually saved, and so the .grad() call inside must trigger a recomputation.
    #
    # During recomputation the "inner pack hook" has two responsibilities:
    #
    # 1) As usual, populating the WeakKeyDictionary storing recomputed tensors
    # 2) Pack the actual tensor (detached) so that one may perform backward on the
    #    recomputed graph. The tensors saved to this graph will live until the end
    #    of recomputation, or die earlier if someone performs backward with
    #    retain_graph=False.
    #
    # More generally performing backward on the recomputed graph occurs in the
    # following cases:
    # - If backward is performed inside forward,
    #   - During the original forward IF early-stop is disabled
    #   - During the original backward
    # - If there are multiple .grad()/.backward() calls, we would perform backward
    #   on the recomputed graph even if early-stop is enabled (see the example below)
    #
    # [ retain_graph is False ]
    #
    # The example below shows what happens if during recomputation we find that some
    # of the tensors we are trying to recompute have already been cleared.
    #
    # Spoiler: we don't do anything special, we just skip over them!
    #
    # def fn(x):
    #   y = x.sin()                           # (1)
    #   z = y.cos()                           # (2)
    #   gx, = torch.autograd.grad(z, x)       # (3)
    #   return x.cos() * gx                   # (4)
    #
    # out = checkpoint(fn)(inp)
    # out.backward()                          # (5)
    #
    # 1, 2. Don't save x and y since we are inside a checkpoint.
    # 3. Trigger a recompute of fn since x and y weren't saved.
    #    And depending on whether early stop is enabled, either stop at (2) or
    #    continue running the function.
    #    Because we are running backward with retain_graph=False, we clear x and y's
    #    holders.
    # 4. Don't save x since we are inside a checkpoint.
    # 5. Calling backward triggers another recompute of fn. During recompute, we see
    #    that x and y have already been cleared in the original graph as indicated
    #    by holder=None. We skip over them. We still save x at (4) (since its holder
    #    is still alive.)

    _enable_checkpoint_early_stop = True

    @contextlib.contextmanager
    def set_checkpoint_early_stop(enable: bool):
        """Context manager that sets whether checkpoint should stop recomputation
        early.

        By default, non-reentrant checkpoint stops recomputation as soon as it
        has computed all needed Tensors. This context manager can be used to disable
        that feature if it is problematic for your specific application.

        This context manager only needs to be active when forward is run. It does
        not need to be active during backward.

        Example::

        >>> # xdoctest: +SKIP(failing)
        >>> message = "saved tensors default hooks are disabled"
        >>> with set_checkpoint_early_stop(False):
        ...     # Any checkpoint under this context manager will respect this
        ...     # context manager, even if its backward is performed outside.
        ...     out = checkpoint(fn, inputs)
        ...
        >>> out.backward()
        """
        global _enable_checkpoint_early_stop
        try:
            prev = _enable_checkpoint_early_stop
            _enable_checkpoint_early_stop = enable
            yield
        finally:
            _enable_checkpoint_early_stop = prev

    class _Handle:
        pass

    class _Holder:
        def __init__(self):
            self.handles: Dict[int, Optional[_Handle]] = dict()

    class _NoopSaveInputs(torch.autograd.Function):
        @staticmethod
        def forward(*args):
            return torch.empty((0,))

        @staticmethod
        def setup_context(ctx: Any, inputs: Tuple[Any, ...], output: Any) -> None:
            # Only tensors can be saved with ctx.save_for_backward, everything else
            # is captured by get_args, which is saved directly on ctx
            tensor_indices, tensors = zip(
                *[(i, o) for i, o in enumerate(inputs) if isinstance(o, torch.Tensor)]
            )
            idx2saved_idx = {b: a for a, b in enumerate(tensor_indices)}
            # args but with tensors replaced with None as placeholders
            args = [None if isinstance(o, torch.Tensor) else o for o in inputs]

            def get_args(saved_tensors):
                # restore the placeholders with the original tensors grabbed from
                # ctx.saved_tensors (which may be saved on a parent checkpoint if
                # this checkpoint is nested, and that would trigger a recursive
                # unpack!)
                ret = [
                    saved_tensors[idx2saved_idx[i]] if i in tensor_indices else o
                    for i, o in enumerate(args)
                ]
                # grab the tail since we also saved the dummy to avoid having to explicitly
                # handle the case where there are no tensor inputs
                return ret[1:]

            ctx.get_args = get_args
            ctx.save_for_backward(*tensors)

        @staticmethod
        def backward(ctx, *grad_outputs):
            raise AssertionError("Did not expect to backward on this graph")

    class _CheckpointFrame:
        def __init__(self, recompute_fn, early_stop, unpack_error_cb, metadata_fn):
            self.recompute_fn = recompute_fn
            self.input_saver = None
            self.weak_holders: List[ReferenceType] = []
            # We store this as a weakkeydictionary so that in the case of a partial
            # backward, the entries in the dict are cleared alongside the Holder
            # which will be removed when the SavedVariable is cleared.
            self.recomputed: DefaultDict[
                int, weakref.WeakKeyDictionary[_Handle, torch.Tensor]
            ] = defaultdict(weakref.WeakKeyDictionary)
            # We need both recomp_counter and recomputed since they can diverge
            # https://github.com/pytorch/pytorch/pull/90105#discussion_r1135889885
            self.recomp_counter: DefaultDict[int, int] = defaultdict(int)
            self.is_recomputed: DefaultDict[int, bool] = defaultdict(bool)

            # See Rule 5
            self.early_stop = early_stop

            # Debugging
            self.metadata_fn = metadata_fn
            self.unpack_error_cb = unpack_error_cb
            self.x_metadatas = []
            self.forward_completed = False
            self.ignore_saved_mismatch = False

        def check_recomputed_tensors_match(self, gid):
            if self.ignore_saved_mismatch:
                # TODO: we can probably make this check stricter by checking that
                #       the metadata of the first tensors still match.
                return
            # NOTE [ Error handling for checkpoint ]
            #
            # At a high level, we need to check that the tensors saved
            # during original forward matches tensors saved during recompute
            # This means handling 3 cases:
            #
            # 1. During recompute, more tensors were saved.
            #
            #    Usually this is hidden due to the StopRecomputationError
            #    but if early stop is not enabled, or we would have errored
            #    anyway because there aren't enough weak_holders. But we
            #    do want to have a nice error. See the _recomputation_hook
            #    for details.
            if not len(self.weak_holders) == self.recomp_counter[gid]:
                # 2. During recompute, fewer tensors were saved
                #
                # We know that everytime we save something do original forward
                # we append to weak_holder, and every time we save a tensor
                # during recompute we increment recompute_counter.
                raise CheckpointError(
                    "torch.utils.checkpoint: A different number of tensors was saved "
                    "during the original forward and recomputation.\n"
                    f"Number of tensors saved during forward: {len(self.weak_holders)}\n"
                    f"Number of tensors saved during recomputation: {self.recomp_counter[gid]}"
                )

            # 3. During recompute, the same tensors were saved, but they
            #    have different metadata
            nb_meta_different = []
            for idx, weak_holder in enumerate(self.weak_holders):
                holder = weak_holder()
                if holder is None:
                    continue
                # We've seen all holders since we iterate over them in order
                # For every holder that is still alive now, it must've been
                # alive when we saw it during recompute, therefore, the
                # gid must be set.
                _internal_assert(gid in holder.handles)
                # We know this is the first unpack, so it couldn't have been set
                # to None yet.
                _internal_assert(holder.handles[gid] is not None)
                # We always set these together in the recomputation hook
                _internal_assert(holder.handles[gid] in self.recomputed[gid])
                # see pack hook, x_metadata is 1:1 with weak_holders.
                x_meta = self.x_metadatas[idx]
                recomputed_x = self.recomputed[gid][holder.handles[gid]]
                if x_meta != self.metadata_fn(recomputed_x):
                    nb_meta_different.append((idx, x_meta, self.metadata_fn(recomputed_x)))

            if len(nb_meta_different) > 0:
                mismatched_tensors = ""
                for idx, x_meta, recomputed_meta in nb_meta_different:
                    mismatched_tensors += (
                        f"tensor at position {idx}:\n"
                        f"saved metadata: {x_meta}\n"
                        f"recomputed metadata: {recomputed_meta}\n"
                    )
                raise CheckpointError(
                    "torch.utils.checkpoint: Recomputed values for the following tensors "
                    "have different metadata than during the forward pass.\n"
                    f"{mismatched_tensors}"
                )

    _checkpoint_error_template = """ \
    An error happened while unpacking tensors; dumping logs of latest computation
    because you passed `debug=True` to `torch.utils.checkpoint.checkpoint()`.
    Scroll all the way down for guidance on how to navigate these logs.

    +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~+
    |        1. Stack traces of the operators that ran in the original forward     |
    +------------------------------------------------------------------------------+

    {forward_traces}
    +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~+
    |        2. Stack traces of the operators that ran during recomputation        |
    +------------------------------------------------------------------------------+

    {recompute_traces}
    +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~+
    |       3. Log of operators in the original forward and recomputation          |
    +------------------------------------------------------------------------------+
    (Scroll up to correlate stack traces with each operation listed below. This
     helps identify their source in the code.)

    IMPORTANT: Differences in "detach" calls between the original forward and the
               recomputation are expected. They are introduced by the checkpointing
               mechanism and can be ignored.

    Operations executed during the original forward:

    {forward_ops}

    Operations executed during recomputation:

    {recompute_ops}

    +------------------------------------------------------------------------------+
     ERROR: Detected non-determinism while running activation checkpointing

     You are seeing this error because you passed `debug=True` to checkpoint and
     tensors to be saved during the original forward and differ between those saved
     during recomputation. This can happen if different operators were ran in the
     original forward and in the recomputation.

     To identify where the mismatch may be coming from, you can do the following:

     1) Compare the operators ran during original forward and recomputation to
        see where they differ. These operators are printed above in the order they
        were executed.

     2) Review the stack trace for each operator to locate its invocation source.
        Each operator's stack trace is printed in their execution order.

     Note that the logs can be quite long. Here's how they are structured:
     (Tip: you can Ctrl-f for these headers)

     1. Stack traces of the operators that ran in the original forward
     2. Stack traces of the operators that ran during recomputation
     3. Log of operators in the original forward and recomputation
     4. Error message                                             <--- You are here
    --------------------------------------------------------------------------------
    """

    class CheckpointError(RuntimeError):
        pass

    def _get_debug_context_and_cb() -> Tuple[Callable[[], Any], Callable[[CheckpointError], None]]:
        # This function returns the context_fn and error_cb to be used by the
        # checkpointing mechanism. error_cb is invoked when an error is detected
        # during unpack.

        # record_context_cpp is not support on non-linux non-x86_64 platforms
        cpp_tb = platform.machine() == 'x86_64' and platform.system() == 'Linux'

        class CaptureLogs:
            def __init__(self):
                self.logs = None
                self.tbs = None

            def get_context_manager(self):
                @contextlib.contextmanager
                def logging_mode():
                    with LoggingTensorMode(), \
                         capture_logs(True, python_tb=True, script_tb=True, cpp_tb=cpp_tb) as logs_and_tb:
                        self.logs, self.tbs = logs_and_tb
                        yield logs_and_tb
                return logging_mode()

        capture_logs_fwd = CaptureLogs()
        capture_logs_recompute = CaptureLogs()

        def unpack_error_cb(e: CheckpointError):
            def get_str_tb(label, capture_logs):
                out = ""
                total_len = len(capture_logs.logs)
                for i, (log, tb) in enumerate(zip(capture_logs.logs, capture_logs.tbs)):
                    out += f"{log}   ({i + 1} of {total_len} in {label})\n\n"
                    found_torch_dispatch = False
                    for line in tb:
                        # Start printing stack trace only after __torch_dispatch__ is found
                        is_torch_dispatch = line['name'] == '__torch_dispatch__'
                        if not found_torch_dispatch and not is_torch_dispatch:
                            continue
                        elif is_torch_dispatch:
                            found_torch_dispatch = True
                            continue
                        out += f"{line['filename']}:{line['line']}:{line['name']}\n"
                    out += "\n\n"
                return out
            assert capture_logs_fwd.logs is not None
            assert capture_logs_recompute.logs is not None
            raise CheckpointError(
                _checkpoint_error_template.format(
                    forward_traces=get_str_tb("original", capture_logs_fwd),
                    recompute_traces=get_str_tb("recompute", capture_logs_recompute),
                    forward_ops="\n".join(capture_logs_fwd.logs),
                    recompute_ops="\n".join(capture_logs_recompute.logs)
                )
            ) from e

        def context_fn():
            return capture_logs_fwd.get_context_manager(), capture_logs_recompute.get_context_manager()

        return context_fn, unpack_error_cb

    def _default_meta_extractor(x: torch.Tensor) -> Dict[str, Any]:
        # These properties are fast to check, easy to understand
        return {
            "shape": x.shape,
            "dtype": x.dtype,
            "device": x.device
        }

    _allowed_determinism_checks_to_fns: Dict[str, Callable[[torch.Tensor], Any]] = {
        _DEFAULT_DETERMINISM_MODE: _default_meta_extractor,
        "none": lambda _: None,
    }

    # See Rule 5
    class _StopRecomputationError(Exception):
        pass

    class _recomputation_hook(torch.autograd.graph.saved_tensors_hooks):
        def __init__(self, target_frame_ref: ReferenceType, gid: int):
            def pack_hook(x):
                target_frame = target_frame_ref()
                assert target_frame is not None  # appease mypy
                recomp_idx = target_frame.recomp_counter[gid]
                target_frame.recomp_counter[gid] += 1

                if recomp_idx >= len(target_frame.weak_holders):
                    assert not target_frame.early_stop
                    if not target_frame.forward_completed:
                        # We run into this case when early stop is not enabled and do
                        # grad within checkpoint.
                        # We need to set this flag, so we don't error out later when
                        # we check if the number of tensors saved during forward and
                        # recomputation match.
                        target_frame.ignore_saved_mismatch = True
                        return x.detach()
                    raise CheckpointError(
                        "torch.utils.checkpoint: trying to save more tensors during "
                        "recomputation than during the original forward pass."
                    )

                holder = target_frame.weak_holders[recomp_idx]()

                # This holder may have been cleared because someone may have called
                # backward within forward. If so, we don't need to save.
                if holder is not None:
                    _internal_assert(holder.handles.get(gid, None) is None)
                    holder.handles[gid] = _Handle()
                    target_frame.recomputed[gid][holder.handles[gid]] = x.detach()

                if target_frame.early_stop and target_frame.recomp_counter[gid] == len(
                    target_frame.weak_holders
                ):
                    raise _StopRecomputationError()
                # See Rule 6: [ retain_graph is True ] above
                return x.detach()

            def unpack_hook(x):
                # See Rule 6: [ retain_graph is True ] above for an example of when
                # the graph created during recomputation could be backwarded.
                return x

            super().__init__(pack_hook, unpack_hook)

    class _checkpoint_hook(torch.autograd.graph.saved_tensors_hooks):
        def __init__(self, frame):
            def pack_hook(x):
                # See Rule 4 above
                holder = _Holder()
                frame.weak_holders.append(weakref.ref(holder))
                # Save metadata to detect non-determinism
                if frame.metadata_fn is not None:
                    with torch.no_grad():
                        frame.x_metadatas.append(frame.metadata_fn(x))
                return holder

            def unpack_hook(holder):
                gid = torch._C._current_graph_task_id()
                if gid == -1:
                    # generate a temporary id if we trigger unpack outside of a backward call
                    gid = int(uuid.uuid4())

                if not frame.is_recomputed[gid]:
                    ctx = frame.input_saver.grad_fn
                    args = ctx.get_args(ctx.saved_tensors)

                    try:
                        with _recomputation_hook(
                            weakref.ref(frame), gid
                        ), torch.autograd.enable_grad():
                            frame.recompute_fn(*args)
                    except _StopRecomputationError:
                        pass
                    frame.is_recomputed[gid] = True
                    frame.check_recomputed_tensors_match(gid)

                _internal_assert(gid in holder.handles)

                if holder.handles[gid] is None:
                    raise CheckpointError(
                        "torch.utils.checkpoint: Unpack is being triggered for a tensor that was already "
                        "unpacked once. If you are calling ctx.saved_tensors in backward, make sure to do "
                        "so only once. Otherwise please open an issue with details on your use case."
                    )
                _internal_assert(holder.handles[gid] in frame.recomputed[gid])
                ret = frame.recomputed[gid][holder.handles[gid]]
                holder.handles[gid] = None
                return ret

            if frame.unpack_error_cb is not None:
                def unpack_hook_with_error_cb(holder):
                    try:
                        return unpack_hook(holder)
                    except CheckpointError as e:
                        frame.unpack_error_cb(e)
                super().__init__(pack_hook, unpack_hook_with_error_cb)
            else:
                super().__init__(pack_hook, unpack_hook)

    # NB: this helper wraps fn before calling checkpoint_impl. kwargs and
    #     saving/restoring of global state is handled here.

    def _checkpoint_without_reentrant_generator(
        fn,
        preserve_rng_state=True,
        context_fn: Callable[[], Tuple[ContextManager, ContextManager]] = noop_context_fn,
        determinism_check: str = _DEFAULT_DETERMINISM_MODE,
        debug: bool = False,
        *args,
        **kwargs
    ):
        """Checkpointing without reentrant autograd
        Args:
            function: describes what to run in the forward pass of the model or
                part of the model. It should also know how to handle the inputs
                passed as the tuple. For example, in LSTM, if user passes
                ``(activation, hidden)``, :attr:`function` should correctly use the
                first input as ``activation`` and the second input as ``hidden``
            preserve_rng_state(bool, optional):  Omit stashing and restoring
                the RNG state during each checkpoint.
                Default: ``True``
            context_fn(Callable, optional): A callable returning a tuple of two
                context managers. The function and its recomputation will be run
                under the first and second context managers respectively.
            determinism_check(str, optional): A string specifying the determinism
                check to perform. By default it is set to ``"default"`` which
                compares the shapes, dtypes, and devices of the recomputed tensors
                against those the saved tensors. To turn off this check, specify
                ``"none"``. Currently these are the only two supported values.
                Please open an issue if you would like to see more determinism
                checks.
            debug(bool, optional): If ``True``, error messages will also include
                a trace of the operators ran during the original forward computation
                as well as the recomputation.
            *args: Arguments to pass in to the given ``function``.
            **kwargs: Keyword arguments to pass into the given ``function``.
        """
        unpack_error_cb = None

        if debug:
            if context_fn != noop_context_fn:
                raise ValueError(
                    "debug=True is incompatible with non-default context_fn"
                )
            context_fn, unpack_error_cb = _get_debug_context_and_cb()

        if determinism_check in _allowed_determinism_checks_to_fns:
            metadata_fn = _allowed_determinism_checks_to_fns[determinism_check]
        else:
            raise ValueError(
                f"determinism_check should be one of {list(_allowed_determinism_checks_to_fns.keys())}, "
                f"but got {determinism_check}"
            )

        device = _infer_device_type(*args)
        device_module = _get_device_module(device)
        forward_context, recompute_context = context_fn()
        # Accommodates the (remote) possibility that autocast is enabled for cpu AND gpu.
        device_autocast_kwargs, cpu_autocast_kwargs = _get_autocast_kwargs(device=device)

        if preserve_rng_state:
            fwd_cpu_state = torch.get_rng_state()
            # Don't eagerly initialize the cuda context by accident.
            # (If the user intends that the context is initialized later, within their
            # run_function, we SHOULD actually stash the cuda state here.  Unfortunately,
            # we have no way to anticipate this will happen before we run the function.
            # If they do so, we raise an error.)
            had_device_in_fwd = False
            if getattr(device_module, "_initialized", False):
                had_device_in_fwd = True
                fwd_devices, fwd_device_states = get_device_states(*args)

        def recompute_fn(*inputs):
            kwargs, *args = inputs
            # This will be called later during recomputation. This wrapping enables
            # the necessary global state to be captured.
            rng_devices = []
            if preserve_rng_state and had_device_in_fwd:
                rng_devices = fwd_devices
            with torch.random.fork_rng(
                devices=rng_devices, enabled=preserve_rng_state, device_type=device
            ):
                if preserve_rng_state:
                    torch.set_rng_state(fwd_cpu_state)
                    if had_device_in_fwd:
                        set_device_states(fwd_devices, fwd_device_states)

                device_autocast_ctx = device_module.amp.autocast(
                    **device_autocast_kwargs
                ) if _supports_autocast(device) else contextlib.nullcontext()
                with device_autocast_ctx, torch.cpu.amp.autocast(**cpu_autocast_kwargs), \
                     recompute_context:
                    fn(*args, **kwargs)

        new_frame = _CheckpointFrame(
            recompute_fn,
            _enable_checkpoint_early_stop,
            unpack_error_cb,
            metadata_fn
        )
        dummy = torch.empty((0,), requires_grad=True)
        new_frame.input_saver = _NoopSaveInputs.apply(dummy, kwargs, *args)

        # When ambient grad_mode is False
        if new_frame.input_saver.grad_fn is None:
            yield
            return

        with _checkpoint_hook(new_frame), forward_context:
            yield
        new_frame.forward_completed = True

        if getattr(device_module, "_initialized", False) and \
           preserve_rng_state and not had_device_in_fwd:
            # Device was not initialized before running the forward, so we didn't
            # stash the device state.
            raise RuntimeError(
                "PyTorch's device state was initialized in the forward pass "
                "of a Checkpoint, which is not allowed. Please open an issue "
                "if you need this feature."
            )

        return

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