NestedIOFunction

class torch.autograd.function.NestedIOFunction(*args, **kwargs)

This class is here only for backward compatibility reasons. Use Function instead of this for any new use case.

backward(*gradients)

Shared backward utility.

Return type:

Any

backward_extended(*grad_output)

User defined backward.

forward(*args)

Shared forward utility.

Return type:

Any

forward_extended(*input)

User defined forward.

property input_grad_buffers: tuple[Tensor | None, ...]

Return existing buffers for accumulating gradients of this Function’s inputs.

Each entry corresponds to an argument passed to forward(), in the same order. Each entry is None or the autograd engine’s current InputBuffer for that input. A non-None buffer contains gradient contributions already produced during the current backward. A custom backward may accumulate its contribution directly into the buffer and return None for that input, fusing gradient computation with accumulation and avoiding a separate gradient tensor.

Availability follows backward execution order and is independent for each input. An entry is None when no earlier producer has contributed to that input, or when the existing buffer is aliased or cannot safely be updated in place.

For example, x has another forward use while weight does not. The custom backward conditionally fuses accumulation only for grad_x:

>>> class Matmul(torch.autograd.Function):
>>>     @staticmethod
>>>     def forward(ctx, x, weight):
>>>         ctx.save_for_backward(x, weight)
>>>         return x @ weight
>>>
>>>     @staticmethod
>>>     def backward(ctx, grad_output):
>>>         x, weight = ctx.saved_tensors
>>>         x_buffer, _ = ctx.input_grad_buffers
>>>         if x_buffer is not None:
>>>             # Computes grad_x and adds it to the existing buffer.
>>>             matmul_backward_input_acc(
>>>                 grad_output, weight, acc_into=x_buffer
>>>             )
>>>             grad_x = None
>>>         else:
>>>             grad_x = matmul_backward_input(grad_output, weight)
>>>         grad_weight = matmul_backward_weight(grad_output, x)
>>>         return grad_x, grad_weight
>>>
>>> loss = Matmul.apply(x, weight).sum() + other_op(x).sum()

If other_op produces its contribution first, x_buffer can expose that partial sum. If Matmul runs first, x_buffer is None and it returns a separate tensor instead. The fallback lets the function work under either ordering.

Warning

A returned buffer is valid only while the current custom backward invocation is running. Mutate it synchronously and do not retain it. A later producer may replace the engine’s buffer, making a retained tensor stale.

After receiving a non-None buffer, calling backward or grad before the custom backward returns raises an error.

All engine-scheduled producers that use or subsequently update an exposed buffer must execute on the same device, autograd engine thread, and stream.

This property is available only while a Python custom backward is executing during an eager, first-order backward(), torch.autograd.backward(), or torch.autograd.grad() call. It is unavailable with create_graph=True, anomaly detection, a post-hook on the producing autograd node, or stale capture stream overrides.

Note

For a leaf input, a non-None entry exposes its execution-local InputBuffer, not its existing .grad. All contributions are first combined in that buffer. During backward, AccumulateGrad then runs once with the completed buffer to update .grad and run its usual hooks. torch.autograd.grad() instead returns the completed buffer without updating .grad.

During backward, a custom backward that instead accumulates directly into a leaf .grad and returns None does not use this interface. It is responsible for managing .grad state, including initialization and lifetime, synchronization with all other producers, and any AccumulateGrad hook behavior bypassed by the direct write.

static jvp(ctx, *grad_inputs)

Define a formula for differentiating the operation with forward mode automatic differentiation.

This function is to be overridden by all subclasses. It must accept a context ctx as the first argument, followed by as many inputs as the forward() got (None will be passed in for non tensor inputs of the forward function), and it should return as many tensors as there were outputs to forward(). Each argument is the gradient w.r.t the given input, and each returned value should be the gradient w.r.t. the corresponding output. If an output is not a Tensor or the function is not differentiable with respect to that output, you can just pass None as a gradient for that input.

You can use the ctx object to pass any value from the forward to this functions.

Return type:

Any

mark_dirty(*args, **kwargs)

See Function.mark_dirty().

mark_non_differentiable(*args, **kwargs)

See Function.mark_non_differentiable().

save_for_backward(*args)

See Function.save_for_backward().

save_for_forward(*tensors)

Save given tensors for a future call to jvp().

save_for_forward should be called at most once, in either the setup_context() or forward() methods, and all arguments should be tensors.

In jvp(), saved objects can be accessed through the saved_tensors attribute.

Arguments can also be None. This is a no-op.

See Extending torch.autograd for more details on how to use this method.

Example:

>>> class Func(torch.autograd.Function):
>>>     @staticmethod
>>>     def forward(ctx, x: torch.Tensor, y: torch.Tensor, z: int):
>>>         ctx.save_for_backward(x, y)
>>>         ctx.save_for_forward(x, y)
>>>         ctx.z = z
>>>         return x * y * z
>>>
>>>     @staticmethod
>>>     def jvp(ctx, x_t, y_t, _):
>>>         x, y = ctx.saved_tensors
>>>         z = ctx.z
>>>         return z * (y * x_t + x * y_t)
>>>
>>>     @staticmethod
>>>     def vjp(ctx, grad_out):
>>>         x, y = ctx.saved_tensors
>>>         z = ctx.z
>>>         return z * grad_out * y, z * grad_out * x, None
>>>
>>>     a = torch.tensor(1., requires_grad=True, dtype=torch.double)
>>>     t = torch.tensor(1., dtype=torch.double)
>>>     b = torch.tensor(2., requires_grad=True, dtype=torch.double)
>>>     c = 4
>>>
>>>     with fwAD.dual_level():
>>>         a_dual = fwAD.make_dual(a, t)
>>>         d = Func.apply(a_dual, b, c)

property saved_tensors

See Function.saved_tensors().

set_materialize_grads(value)

Set whether to materialize grad tensors. Default is True.

This should be called only from either the setup_context() or forward() methods.

If True, undefined grad tensors will be expanded to tensors full of zeros prior to calling the backward() and jvp() methods.

Example:

>>> class SimpleFunc(Function):
>>>     @staticmethod
>>>     def forward(ctx, x):
>>>         return x.clone(), x.clone()
>>>
>>>     @staticmethod
>>>     @once_differentiable
>>>     def backward(ctx, g1, g2):
>>>         return g1 + g2  # No check for None necessary
>>>
>>> # We modify SimpleFunc to handle non-materialized grad outputs
>>> class Func(Function):
>>>     @staticmethod
>>>     def forward(ctx, x):
>>>         ctx.set_materialize_grads(False)
>>>         ctx.save_for_backward(x)
>>>         return x.clone(), x.clone()
>>>
>>>     @staticmethod
>>>     @once_differentiable
>>>     def backward(ctx, g1, g2):
>>>         x, = ctx.saved_tensors
>>>         grad_input = torch.zeros_like(x)
>>>         if g1 is not None:  # We must check for None now
>>>             grad_input += g1
>>>         if g2 is not None:
>>>             grad_input += g2
>>>         return grad_input
>>>
>>> a = torch.tensor(1., requires_grad=True)
>>> b, _ = Func.apply(a)  # induces g2 to be undefined

set_output_grad_dtype(*dtypes)

Declare the gradient dtype for each of this Function’s outputs.

This should be called at most once, from either the setup_context() or forward() methods. The number of declarations must match the number of returned values, and each argument corresponds positionally to the output at the same index.

For each output, pass the dtype your backward should receive its gradient in:

• Pass a torch.dtype and the engine guarantees the gradient handed to backward has that dtype. This is only valid for a differentiable Tensor output.

• Pass None and the gradient is handed to backward with whatever dtype it already has. This is also the only valid choice for a non-Tensor or non-differentiable output, which has no gradient.

• Omit this call (or pass the output’s own dtype) and the gradient is handed to backward in the output’s dtype, which is the default.

For example:

>>> @staticmethod
>>> def forward(ctx, x):
>>>     t1 = x.sin()
>>>     t2 = x.cos()
>>>     t3 = x.tan()
>>>     ctx.set_output_grad_dtype(torch.float32, t2.dtype, None, None)
>>>     return t1, t2, t3, "not a tensor"

This ensures that backward receives t1’s gradient in float32, keeps the default behavior for t2’s gradient via t2.dtype, passes t3’s gradient through uncast with None, and uses None as the placeholder for the trailing non-Tensor output.

static setup_context(ctx, inputs, output)

There are two ways to define the forward pass of an autograd.Function.

Either:

1. Override forward with the signature forward(ctx, *args, **kwargs). setup_context is not overridden. Setting up the ctx for backward happens inside the forward.

2. Override forward with the signature forward(*args, **kwargs) and override setup_context. Setting up the ctx for backward happens inside setup_context (as opposed to inside the forward)

See torch.autograd.Function.forward() and Extending torch.autograd for more details.

Return type:

Any

static vjp(ctx, *grad_outputs)

Define a formula for differentiating the operation with backward mode automatic differentiation.

This function is to be overridden by all subclasses. (Defining this function is equivalent to defining the vjp function.)

It must accept a context ctx as the first argument, followed by as many outputs as the forward() returned (None will be passed in for non tensor outputs of the forward function), and it should return as many tensors, as there were inputs to forward(). Each argument is the gradient w.r.t the given output, and each returned value should be the gradient w.r.t. the corresponding input. If an input is not a Tensor or is a Tensor not requiring grads, you can just pass None as a gradient for that input.

The strides of the gradients passed to backward() are undefined: they are not guaranteed to be contiguous or to match the strides of the corresponding forward outputs, so implementations must not assume a particular memory layout.

The context can be used to retrieve tensors saved during the forward pass. It also has an attribute ctx.needs_input_grad as a tuple of booleans representing whether each input needs gradient. E.g., backward() will have ctx.needs_input_grad[0] = True if the first input to forward() needs gradient computed w.r.t. the output.

Return type:

Any

static vmap(info, in_dims, *args)

Define the behavior for this autograd.Function underneath torch.vmap().

For a torch.autograd.Function() to support torch.vmap(), you must either override this static method, or set generate_vmap_rule to True (you may not do both).

If you choose to override this staticmethod: it must accept

• an info object as the first argument. info.batch_size specifies the size of the dimension being vmapped over, while info.randomness is the randomness option passed to torch.vmap().

• an in_dims tuple as the second argument. For each arg in args, in_dims has a corresponding Optional[int]. It is None if the arg is not a Tensor or if the arg is not being vmapped over, otherwise, it is an integer specifying what dimension of the Tensor is being vmapped over.

• *args, which is the same as the args to forward().

The return of the vmap staticmethod is a tuple of (output, out_dims). Similar to in_dims, out_dims should be of the same structure as output and contain one out_dim per output that specifies if the output has the vmapped dimension and what index it is in.

Please see Extending torch.func with autograd.Function for more details.