Automatic differentiation package  torch.autograd¶
torch.autograd
provides classes and functions implementing automatic
differentiation of arbitrary scalar valued functions. It requires minimal
changes to the existing code  you only need to declare Tensor
s
for which gradients should be computed with the requires_grad=True
keyword.

torch.autograd.
backward
(tensors, grad_tensors=None, retain_graph=None, create_graph=False, grad_variables=None)[source]¶ Computes the sum of gradients of given tensors w.r.t. graph leaves.
The graph is differentiated using the chain rule. If any of
tensors
are nonscalar (i.e. their data has more than one element) and require gradient, then the Jacobianvector product would be computed, in this case the function additionally requires specifyinggrad_tensors
. It should be a sequence of matching length, that contains the “vector” in the Jacobianvector product, usually the gradient of the differentiated function w.r.t. corresponding tensors (None
is an acceptable value for all tensors that don’t need gradient tensors).This function accumulates gradients in the leaves  you might need to zero them before calling it.
 Parameters
tensors (sequence of Tensor) – Tensors of which the derivative will be computed.
grad_tensors (sequence of (Tensor or None)) – The “vector” in the Jacobianvector product, usually gradients w.r.t. each element of corresponding tensors. None values can be specified for scalar Tensors or ones that don’t require grad. If a None value would be acceptable for all grad_tensors, then this argument is optional.
retain_graph (bool, optional) – If
False
, the graph used to compute the grad will be freed. Note that in nearly all cases setting this option toTrue
is not needed and often can be worked around in a much more efficient way. Defaults to the value ofcreate_graph
.create_graph (bool, optional) – If
True
, graph of the derivative will be constructed, allowing to compute higher order derivative products. Defaults toFalse
.

torch.autograd.
grad
(outputs, inputs, grad_outputs=None, retain_graph=None, create_graph=False, only_inputs=True, allow_unused=False)[source]¶ Computes and returns the sum of gradients of outputs w.r.t. the inputs.
grad_outputs
should be a sequence of length matchingoutput
containing the “vector” in Jacobianvector product, usually the precomputed gradients w.r.t. each of the outputs. If an output doesn’t require_grad, then the gradient can beNone
).If
only_inputs
isTrue
, the function will only return a list of gradients w.r.t the specified inputs. If it’sFalse
, then gradient w.r.t. all remaining leaves will still be computed, and will be accumulated into their.grad
attribute. Parameters
outputs (sequence of Tensor) – outputs of the differentiated function.
inputs (sequence of Tensor) – Inputs w.r.t. which the gradient will be returned (and not accumulated into
.grad
).grad_outputs (sequence of Tensor) – The “vector” in the Jacobianvector product. Usually gradients w.r.t. each output. None values can be specified for scalar Tensors or ones that don’t require grad. If a None value would be acceptable for all grad_tensors, then this argument is optional. Default: None.
retain_graph (bool, optional) – If
False
, the graph used to compute the grad will be freed. Note that in nearly all cases setting this option toTrue
is not needed and often can be worked around in a much more efficient way. Defaults to the value ofcreate_graph
.create_graph (bool, optional) – If
True
, graph of the derivative will be constructed, allowing to compute higher order derivative products. Default:False
.allow_unused (bool, optional) – If
False
, specifying inputs that were not used when computing outputs (and therefore their grad is always zero) is an error. Defaults toFalse
.
Functional higher level API¶
Warning
This API is experimental.
This section contains the higher level API for the autograd that builds on the basic API above and allows you to compute jacobians, hessians, etc.
This API works with userprovided functions that take only Tensors as input and return
only Tensors.
If your function takes other arguments that are not Tensors or Tensors for which you don’t require gradients,
you can use a lambda to capture them.
For example, for a function f
that takes three inputs, a Tensor for which we want the jacobian, another
tensor that should be considered constant and a boolean flag as f(input, constant, flag=flag)
you can use it as functional.jacobian(lambda x: f(x, constant, flag=flag), input)
.

torch.autograd.functional.
jacobian
(func, inputs, create_graph=False, strict=False)[source]¶ Function that computes the Jacobian of a given function.
 Parameters
func (function) – a Python function that takes Tensor inputs and returns a tuple of Tensors or a Tensor.
inputs (tuple of Tensors or Tensor) – inputs to the function
func
.create_graph (bool, optional) – If
True
, the Jacobian will be computed in a differentiable manner. Note that whenstrict
isFalse
, the result can not require gradients or be disconnected from the inputs. Defaults toFalse
.strict (bool, optional) – If
True
, an error will be raised when we detect that there exists an input such that all the outputs are independent of it. IfFalse
, we return a Tensor of zeros as the jacobian for said inputs, which is the expected mathematical value. Defaults toFalse
.
 Returns
 Jacobian (Tensor or nested tuple of Tensors) if there are a single input
and output, this will be a single Tensor containing the Jacobian for the linearized inputs and output. If one of the two is a tuple, then the Jacobian will be a tuple of Tensors. If both of them are tuples, then the Jacobian will be a tuple of tuple of Tensors where
Jacobian[i][j]
will contain the Jacobian of the ``i``th output and ``j``th input and will have as size the concatenation of the sizes of the corresponding output and the corresponding input.
Example:
>>> def exp_reducer(x): ... return x.exp().sum(dim=1) >>> inputs = torch.rand(2, 2) >>> jacobian(exp_reducer, inputs) tensor([[[1.4917, 2.4352], [0.0000, 0.0000]], [[0.0000, 0.0000], [2.4369, 2.3799]]]) >>> jacobian(exp_reducer, inputs, create_graph=True) tensor([[[1.4917, 2.4352], [0.0000, 0.0000]], [[0.0000, 0.0000], [2.4369, 2.3799]]], grad_fn=<ViewBackward>) >>> def exp_adder(x, y): ... return 2 * x.exp() + 3 * y >>> inputs = (torch.rand(2), torch.rand(2)) >>> jacobian(exp_adder, inputs) (tensor([[2.8052, 0.0000], [0.0000, 3.3963]]), tensor([[3., 0.], [0., 3.]]))

torch.autograd.functional.
hessian
(func, inputs, create_graph=False, strict=False)[source]¶ Function that computes the Hessian of a given scalar function.
 Parameters
func (function) – a Python function that takes Tensor inputs and returns a Tensor with a single element.
inputs (tuple of Tensors or Tensor) – inputs to the function
func
.create_graph (bool, optional) – If
True
, the Hessian will be computed in a differentiable manner. Note that whenstrict
isFalse
, the result can not require gradients or be disconnected from the inputs. Defaults toFalse
.strict (bool, optional) – If
True
, an error will be raised when we detect that there exists an input such that all the outputs are independent of it. IfFalse
, we return a Tensor of zeros as the hessian for said inputs, which is the expected mathematical value. Defaults toFalse
.
 Returns
 Hessian (Tensor or a tuple of tuple of Tensors) if there are a single input,
this will be a single Tensor containing the Hessian for the input. If it is a tuple, then the Hessian will be a tuple of tuples where
Hessian[i][j]
will contain the Hessian of the ``i``th input and ``j``th input with size the sum of the size of the ``i``th input plus the size of the ``j``th input.
Example:
>>> def pow_reducer(x): ... return x.pow(3).sum() >>> inputs = torch.rand(2, 2) >>> hessian(pow_reducer, inputs) tensor([[[[5.2265, 0.0000], [0.0000, 0.0000]], [[0.0000, 4.8221], [0.0000, 0.0000]]], [[[0.0000, 0.0000], [1.9456, 0.0000]], [[0.0000, 0.0000], [0.0000, 3.2550]]]]) >>> hessian(pow_reducer, inputs, create_graph=True) tensor([[[[5.2265, 0.0000], [0.0000, 0.0000]], [[0.0000, 4.8221], [0.0000, 0.0000]]], [[[0.0000, 0.0000], [1.9456, 0.0000]], [[0.0000, 0.0000], [0.0000, 3.2550]]]], grad_fn=<ViewBackward>) >>> def pow_adder_reducer(x, y): ... return (2 * x.pow(2) + 3 * y.pow(2)).sum() >>> inputs = (torch.rand(2), torch.rand(2)) >>> hessian(pow_adder_reducer, inputs) ((tensor([[4., 0.], [0., 4.]]), tensor([[0., 0.], [0., 0.]])), (tensor([[0., 0.], [0., 0.]]), tensor([[6., 0.], [0., 6.]])))

torch.autograd.functional.
vjp
(func, inputs, v=None, create_graph=False, strict=False)[source]¶ Function that computes the dot product between a vector
v
and the Jacobian of the given function at the point given by the inputs. Parameters
func (function) – a Python function that takes Tensor inputs and returns a tuple of Tensors or a Tensor.
inputs (tuple of Tensors or Tensor) – inputs to the function
func
.v (tuple of Tensors or Tensor) – The vector for which the vector Jacobian product is computed. Must be the same size as the output of
func
. This argument is optional whenfunc
’s output contains a single element and (if it is not provided) will be set as a Tensor containing a single1
.create_graph (bool, optional) – If
True
, both the output and result will be computed in a differentiable way. Note that whenstrict
isFalse
, the result can not require gradients or be disconnected from the inputs. Defaults toFalse
.strict (bool, optional) – If
True
, an error will be raised when we detect that there exists an input such that all the outputs are independent of it. IfFalse
, we return a Tensor of zeros as the vjp for said inputs, which is the expected mathematical value. Defaults toFalse
.
 Returns
output of
func(inputs)
vjp (tuple of Tensors or Tensor): result of the dot product with the same shapeas the inputs.
 Return type
func_output (tuple of Tensors or Tensor)
Example:
>>> def exp_reducer(x): ... return x.exp().sum(dim=1) >>> inputs = torch.rand(4, 4) >>> v = torch.ones(4) >>> vjp(exp_reducer, inputs, v) (tensor([5.7817, 7.2458, 5.7830, 6.7782]), tensor([[1.4458, 1.3962, 1.3042, 1.6354], [2.1288, 1.0652, 1.5483, 2.5035], [2.2046, 1.1292, 1.1432, 1.3059], [1.3225, 1.6652, 1.7753, 2.0152]])) >>> vjp(exp_reducer, inputs, v, create_graph=True) (tensor([5.7817, 7.2458, 5.7830, 6.7782], grad_fn=<SumBackward1>), tensor([[1.4458, 1.3962, 1.3042, 1.6354], [2.1288, 1.0652, 1.5483, 2.5035], [2.2046, 1.1292, 1.1432, 1.3059], [1.3225, 1.6652, 1.7753, 2.0152]], grad_fn=<MulBackward0>)) >>> def adder(x, y): ... return 2 * x + 3 * y >>> inputs = (torch.rand(2), torch.rand(2)) >>> v = torch.ones(2) >>> vjp(adder, inputs, v) (tensor([2.4225, 2.3340]), (tensor([2., 2.]), tensor([3., 3.])))

torch.autograd.functional.
jvp
(func, inputs, v=None, create_graph=False, strict=False)[source]¶ Function that computes the dot product between the Jacobian of the given function at the point given by the inputs and a vector
v
. Parameters
func (function) – a Python function that takes Tensor inputs and returns a tuple of Tensors or a Tensor.
inputs (tuple of Tensors or Tensor) – inputs to the function
func
.v (tuple of Tensors or Tensor) – The vector for which the Jacobian vector product is computed. Must be the same size as the input of
func
. This argument is optional whenfunc
’s input contains a single element and (if it is not provided) will be set as a Tensor containing a single1
.create_graph (bool, optional) – If
True
, both the output and result will be computed in a differentiable way. Note that whenstrict
isFalse
, the result can not require gradients or be disconnected from the inputs. Defaults toFalse
.strict (bool, optional) – If
True
, an error will be raised when we detect that there exists an input such that all the outputs are independent of it. IfFalse
, we return a Tensor of zeros as the jvp for said inputs, which is the expected mathematical value. Defaults toFalse
.
 Returns
output of
func(inputs)
jvp (tuple of Tensors or Tensor): result of the dot product with the same shapeas the output.
 Return type
func_output (tuple of Tensors or Tensor)
Example:
>>> def exp_reducer(x): ... return x.exp().sum(dim=1) >>> inputs = torch.rand(4, 4) >>> v = torch.ones(4, 4) >>> jvp(exp_reducer, inputs, v) (tensor([6.3090, 4.6742, 7.9114, 8.2106]), tensor([6.3090, 4.6742, 7.9114, 8.2106])) >>> jvp(exp_reducer, inputs, v, create_graph=True) (tensor([6.3090, 4.6742, 7.9114, 8.2106], grad_fn=<SumBackward1>), tensor([6.3090, 4.6742, 7.9114, 8.2106], grad_fn=<SqueezeBackward1>)) >>> def adder(x, y): ... return 2 * x + 3 * y >>> inputs = (torch.rand(2), torch.rand(2)) >>> v = (torch.ones(2), torch.ones(2)) >>> jvp(adder, inputs, v) (tensor([2.2399, 2.5005]), tensor([5., 5.]))
Note:
The jvp is currently computed by using the backward of the backward (sometimes called the double backwards trick) as we don't have support for forward mode AD in PyTorch at the moment.

torch.autograd.functional.
vhp
(func, inputs, v=None, create_graph=False, strict=False)[source]¶ Function that computes the dot product between a vector
v
and the Hessian of a given scalar function at the point given by the inputs. Parameters
func (function) – a Python function that takes Tensor inputs and returns a Tensor with a single element.
inputs (tuple of Tensors or Tensor) – inputs to the function
func
.v (tuple of Tensors or Tensor) – The vector for which the vector Hessian product is computed. Must be the same size as the input of
func
. This argument is optional whenfunc
’s input contains a single element and (if it is not provided) will be set as a Tensor containing a single1
.create_graph (bool, optional) – If
True
, both the output and result will be computed in a differentiable way. Note that whenstrict
isFalse
, the result can not require gradients or be disconnected from the inputs. Defaults toFalse
.strict (bool, optional) – If
True
, an error will be raised when we detect that there exists an input such that all the outputs are independent of it. IfFalse
, we return a Tensor of zeros as the vhp for said inputs, which is the expected mathematical value. Defaults toFalse
.
 Returns
output of
func(inputs)
vhp (tuple of Tensors or Tensor): result of the dot product with the same shapeas the inputs.
 Return type
func_output (tuple of Tensors or Tensor)
Example:
>>> def pow_reducer(x): ... return x.pow(3).sum() >>> inputs = torch.rand(2, 2) >>> v = torch.ones(2, 2) >>> vhp(pow_reducer, inputs, v) (tensor(0.5591), tensor([[1.0689, 1.2431], [3.0989, 4.4456]])) >>> vhp(pow_reducer, inputs, v, create_graph=True) (tensor(0.5591, grad_fn=<SumBackward0>), tensor([[1.0689, 1.2431], [3.0989, 4.4456]], grad_fn=<MulBackward0>)) >>> def pow_adder_reducer(x, y): ... return (2 * x.pow(2) + 3 * y.pow(2)).sum() >>> inputs = (torch.rand(2), torch.rand(2)) >>> v = (torch.zeros(2), torch.ones(2)) >>> vhp(pow_adder_reducer, inputs, v) (tensor(4.8053), (tensor([0., 0.]), tensor([6., 6.])))

torch.autograd.functional.
hvp
(func, inputs, v=None, create_graph=False, strict=False)[source]¶ Function that computes the dot product between the Hessian of a given scalar function and a vector
v
at the point given by the inputs. Parameters
func (function) – a Python function that takes Tensor inputs and returns a Tensor with a single element.
inputs (tuple of Tensors or Tensor) – inputs to the function
func
.v (tuple of Tensors or Tensor) – The vector for which the Hessian vector product is computed. Must be the same size as the input of
func
. This argument is optional whenfunc
’s input contains a single element and (if it is not provided) will be set as a Tensor containing a single1
.create_graph (bool, optional) – If
True
, both the output and result will be computed in a differentiable way. Note that whenstrict
isFalse
, the result can not require gradients or be disconnected from the inputs. Defaults toFalse
.strict (bool, optional) – If
True
, an error will be raised when we detect that there exists an input such that all the outputs are independent of it. IfFalse
, we return a Tensor of zeros as the hvp for said inputs, which is the expected mathematical value. Defaults toFalse
.
 Returns
output of
func(inputs)
hvp (tuple of Tensors or Tensor): result of the dot product with the same shapeas the inputs.
 Return type
func_output (tuple of Tensors or Tensor)
Example:
>>> def pow_reducer(x): ... return x.pow(3).sum() >>> inputs = torch.rand(2, 2) >>> v = torch.ones(2, 2) >>> hvp(pow_reducer, inputs, v) (tensor(0.1448), tensor([[2.0239, 1.6456], [2.4988, 1.4310]])) >>> hvp(pow_reducer, inputs, v, create_graph=True) (tensor(0.1448, grad_fn=<SumBackward0>), tensor([[2.0239, 1.6456], [2.4988, 1.4310]], grad_fn=<MulBackward0>)) >>> def pow_adder_reducer(x, y): ... return (2 * x.pow(2) + 3 * y.pow(2)).sum() >>> inputs = (torch.rand(2), torch.rand(2)) >>> v = (torch.zeros(2), torch.ones(2)) >>> hvp(pow_adder_reducer, inputs, v) (tensor(2.3030), (tensor([0., 0.]), tensor([6., 6.])))
Note:
This function is significantly slower than `vhp` due to backward mode AD constraints. If your functions is twice continuously differentiable, then hvp = vhp.t(). So if you know that your function satisfies this condition, you should use vhp instead that is much faster with the current implementation.
Locally disabling gradient computation¶

class
torch.autograd.
no_grad
[source]¶ Contextmanager that disabled gradient calculation.
Disabling gradient calculation is useful for inference, when you are sure that you will not call
Tensor.backward()
. It will reduce memory consumption for computations that would otherwise have requires_grad=True.In this mode, the result of every computation will have requires_grad=False, even when the inputs have requires_grad=True.
This mode has no effect when using
enable_grad
context manager .This context manager is thread local; it will not affect computation in other threads.
Also functions as a decorator. (Make sure to instantiate with parenthesis.)
Example:
>>> x = torch.tensor([1], requires_grad=True) >>> with torch.no_grad(): ... y = x * 2 >>> y.requires_grad False >>> @torch.no_grad() ... def doubler(x): ... return x * 2 >>> z = doubler(x) >>> z.requires_grad False

class
torch.autograd.
enable_grad
[source]¶ Contextmanager that enables gradient calculation.
Enables gradient calculation, if it has been disabled via
no_grad
orset_grad_enabled
.This context manager is thread local; it will not affect computation in other threads.
Also functions as a decorator. (Make sure to instantiate with parenthesis.)
Example:
>>> x = torch.tensor([1], requires_grad=True) >>> with torch.no_grad(): ... with torch.enable_grad(): ... y = x * 2 >>> y.requires_grad True >>> y.backward() >>> x.grad >>> @torch.enable_grad() ... def doubler(x): ... return x * 2 >>> with torch.no_grad(): ... z = doubler(x) >>> z.requires_grad True

class
torch.autograd.
set_grad_enabled
(mode)[source]¶ Contextmanager that sets gradient calculation to on or off.
set_grad_enabled
will enable or disable grads based on its argumentmode
. It can be used as a contextmanager or as a function.When using
enable_grad
context manager,set_grad_enabled(False)
has no effect.This context manager is thread local; it will not affect computation in other threads.
 Parameters
mode (bool) – Flag whether to enable grad (
True
), or disable (False
). This can be used to conditionally enable gradients.
Example:
>>> x = torch.tensor([1], requires_grad=True) >>> is_train = False >>> with torch.set_grad_enabled(is_train): ... y = x * 2 >>> y.requires_grad False >>> torch.set_grad_enabled(True) >>> y = x * 2 >>> y.requires_grad True >>> torch.set_grad_enabled(False) >>> y = x * 2 >>> y.requires_grad False
Inplace operations on Tensors¶
Supporting inplace operations in autograd is a hard matter, and we discourage their use in most cases. Autograd’s aggressive buffer freeing and reuse makes it very efficient and there are very few occasions when inplace operations actually lower memory usage by any significant amount. Unless you’re operating under heavy memory pressure, you might never need to use them.
Inplace correctness checks¶
All Tensor
s keep track of inplace operations applied to them, and
if the implementation detects that a tensor was saved for backward in one of
the functions, but it was modified inplace afterwards, an error will be raised
once backward pass is started. This ensures that if you’re using inplace
functions and not seeing any errors, you can be sure that the computed
gradients are correct.
Variable (deprecated)¶
Warning
The Variable API has been deprecated: Variables are no longer necessary to
use autograd with tensors. Autograd automatically supports Tensors with
requires_grad
set to True
. Below please find a quick guide on what
has changed:
Variable(tensor)
andVariable(tensor, requires_grad)
still work as expected, but they return Tensors instead of Variables.var.data
is the same thing astensor.data
.Methods such as
var.backward(), var.detach(), var.register_hook()
now work on tensors with the same method names.
In addition, one can now create tensors with requires_grad=True
using factory
methods such as torch.randn()
, torch.zeros()
, torch.ones()
, and others
like the following:
autograd_tensor = torch.randn((2, 3, 4), requires_grad=True)
Tensor autograd functions¶

class
torch.
Tensor

grad
¶ This attribute is
None
by default and becomes a Tensor the first time a call tobackward()
computes gradients forself
. The attribute will then contain the gradients computed and future calls tobackward()
will accumulate (add) gradients into it.

requires_grad
¶ Is
True
if gradients need to be computed for this Tensor,False
otherwise.

is_leaf
¶ All Tensors that have
requires_grad
which isFalse
will be leaf Tensors by convention.For Tensors that have
requires_grad
which isTrue
, they will be leaf Tensors if they were created by the user. This means that they are not the result of an operation and sograd_fn
is None.Only leaf Tensors will have their
grad
populated during a call tobackward()
. To getgrad
populated for nonleaf Tensors, you can useretain_grad()
.Example:
>>> a = torch.rand(10, requires_grad=True) >>> a.is_leaf True >>> b = torch.rand(10, requires_grad=True).cuda() >>> b.is_leaf False # b was created by the operation that cast a cpu Tensor into a cuda Tensor >>> c = torch.rand(10, requires_grad=True) + 2 >>> c.is_leaf False # c was created by the addition operation >>> d = torch.rand(10).cuda() >>> d.is_leaf True # d does not require gradients and so has no operation creating it (that is tracked by the autograd engine) >>> e = torch.rand(10).cuda().requires_grad_() >>> e.is_leaf True # e requires gradients and has no operations creating it >>> f = torch.rand(10, requires_grad=True, device="cuda") >>> f.is_leaf True # f requires grad, has no operation creating it

backward
(gradient=None, retain_graph=None, create_graph=False)[source]¶ Computes the gradient of current tensor w.r.t. graph leaves.
The graph is differentiated using the chain rule. If the tensor is nonscalar (i.e. its data has more than one element) and requires gradient, the function additionally requires specifying
gradient
. It should be a tensor of matching type and location, that contains the gradient of the differentiated function w.r.t.self
.This function accumulates gradients in the leaves  you might need to zero them before calling it.
 Parameters
gradient (Tensor or None) – Gradient w.r.t. the tensor. If it is a tensor, it will be automatically converted to a Tensor that does not require grad unless
create_graph
is True. None values can be specified for scalar Tensors or ones that don’t require grad. If a None value would be acceptable then this argument is optional.retain_graph (bool, optional) – If
False
, the graph used to compute the grads will be freed. Note that in nearly all cases setting this option to True is not needed and often can be worked around in a much more efficient way. Defaults to the value ofcreate_graph
.create_graph (bool, optional) – If
True
, graph of the derivative will be constructed, allowing to compute higher order derivative products. Defaults toFalse
.

detach
()¶ Returns a new Tensor, detached from the current graph.
The result will never require gradient.
Note
Returned Tensor shares the same storage with the original one. Inplace modifications on either of them will be seen, and may trigger errors in correctness checks. IMPORTANT NOTE: Previously, inplace size / stride / storage changes (such as resize_ / resize_as_ / set_ / transpose_) to the returned tensor also update the original tensor. Now, these inplace changes will not update the original tensor anymore, and will instead trigger an error. For sparse tensors: Inplace indices / values changes (such as zero_ / copy_ / add_) to the returned tensor will not update the original tensor anymore, and will instead trigger an error.

detach_
()¶ Detaches the Tensor from the graph that created it, making it a leaf. Views cannot be detached inplace.

register_hook
(hook)[source]¶ Registers a backward hook.
The hook will be called every time a gradient with respect to the Tensor is computed. The hook should have the following signature:
hook(grad) > Tensor or None
The hook should not modify its argument, but it can optionally return a new gradient which will be used in place of
grad
.This function returns a handle with a method
handle.remove()
that removes the hook from the module.Example:
>>> v = torch.tensor([0., 0., 0.], requires_grad=True) >>> h = v.register_hook(lambda grad: grad * 2) # double the gradient >>> v.backward(torch.tensor([1., 2., 3.])) >>> v.grad 2 4 6 [torch.FloatTensor of size (3,)] >>> h.remove() # removes the hook

Function¶

class
torch.autograd.
Function
[source]¶ Records operation history and defines formulas for differentiating ops.
Every operation performed on
Tensor
s creates a new function object, that performs the computation, and records that it happened. The history is retained in the form of a DAG of functions, with edges denoting data dependencies (input < output
). Then, when backward is called, the graph is processed in the topological ordering, by callingbackward()
methods of eachFunction
object, and passing returned gradients on to nextFunction
s.Normally, the only way users interact with functions is by creating subclasses and defining new operations. This is a recommended way of extending torch.autograd.
Examples:
>>> class Exp(Function): >>> >>> @staticmethod >>> def forward(ctx, i): >>> result = i.exp() >>> ctx.save_for_backward(result) >>> return result >>> >>> @staticmethod >>> def backward(ctx, grad_output): >>> result, = ctx.saved_tensors >>> return grad_output * result >>> >>> #Use it by calling the apply method: >>> output = Exp.apply(input)

static
backward
(ctx, *grad_outputs)[source]¶ Defines a formula for differentiating the operation.
This function is to be overridden by all subclasses.
It must accept a context
ctx
as the first argument, followed by as many outputs didforward()
return, and it should return as many tensors, as there were inputs toforward()
. 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.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 havectx.needs_input_grad[0] = True
if the first input toforward()
needs gradient computated w.r.t. the output.

static
forward
(ctx, *args, **kwargs)[source]¶ Performs the operation.
This function is to be overridden by all subclasses.
It must accept a context ctx as the first argument, followed by any number of arguments (tensors or other types).
The context can be used to store tensors that can be then retrieved during the backward pass.

static
Context method mixins¶
When creating a new Function
, the following methods are available to ctx.

class
torch.autograd.function.
_ContextMethodMixin
[source]¶ 
mark_dirty
(*args)[source]¶ Marks given tensors as modified in an inplace operation.
This should be called at most once, only from inside the
forward()
method, and all arguments should be inputs.Every tensor that’s been modified inplace in a call to
forward()
should be given to this function, to ensure correctness of our checks. It doesn’t matter whether the function is called before or after modification.

mark_non_differentiable
(*args)[source]¶ Marks outputs as nondifferentiable.
This should be called at most once, only from inside the
forward()
method, and all arguments should be outputs.This will mark outputs as not requiring gradients, increasing the efficiency of backward computation. You still need to accept a gradient for each output in
backward()
, but it’s always going to be a zero tensor with the same shape as the shape of a corresponding output.This is used e.g. for indices returned from a max
Function
.

save_for_backward
(*tensors)[source]¶ Saves given tensors for a future call to
backward()
.This should be called at most once, and only from inside the
forward()
method.Later, saved tensors can be accessed through the
saved_tensors
attribute. Before returning them to the user, a check is made to ensure they weren’t used in any inplace operation that modified their content.Arguments can also be
None
.

Numerical gradient checking¶

torch.autograd.
gradcheck
(func, inputs, eps=1e06, atol=1e05, rtol=0.001, raise_exception=True, check_sparse_nnz=False, nondet_tol=0.0)[source]¶ Check gradients computed via small finite differences against analytical gradients w.r.t. tensors in
inputs
that are of floating point type and withrequires_grad=True
.The check between numerical and analytical gradients uses
allclose()
.Note
The default values are designed for
input
of double precision. This check will likely fail ifinput
is of less precision, e.g.,FloatTensor
.Warning
If any checked tensor in
input
has overlapping memory, i.e., different indices pointing to the same memory address (e.g., fromtorch.expand()
), this check will likely fail because the numerical gradients computed by point perturbation at such indices will change values at all other indices that share the same memory address. Parameters
func (function) – a Python function that takes Tensor inputs and returns a Tensor or a tuple of Tensors
inputs (tuple of Tensor or Tensor) – inputs to the function
eps (float, optional) – perturbation for finite differences
atol (float, optional) – absolute tolerance
rtol (float, optional) – relative tolerance
raise_exception (bool, optional) – indicating whether to raise an exception if the check fails. The exception gives more information about the exact nature of the failure. This is helpful when debugging gradchecks.
check_sparse_nnz (bool, optional) – if True, gradcheck allows for SparseTensor input, and for any SparseTensor at input, gradcheck will perform check at nnz positions only.
nondet_tol (float, optional) – tolerance for nondeterminism. When running identical inputs through the differentiation, the results must either match exactly (default, 0.0) or be within this tolerance.
 Returns
True if all differences satisfy allclose condition

torch.autograd.
gradgradcheck
(func, inputs, grad_outputs=None, eps=1e06, atol=1e05, rtol=0.001, gen_non_contig_grad_outputs=False, raise_exception=True, nondet_tol=0.0)[source]¶ Check gradients of gradients computed via small finite differences against analytical gradients w.r.t. tensors in
inputs
andgrad_outputs
that are of floating point type and withrequires_grad=True
.This function checks that backpropagating through the gradients computed to the given
grad_outputs
are correct.The check between numerical and analytical gradients uses
allclose()
.Note
The default values are designed for
input
andgrad_outputs
of double precision. This check will likely fail if they are of less precision, e.g.,FloatTensor
.Warning
If any checked tensor in
input
andgrad_outputs
has overlapping memory, i.e., different indices pointing to the same memory address (e.g., fromtorch.expand()
), this check will likely fail because the numerical gradients computed by point perturbation at such indices will change values at all other indices that share the same memory address. Parameters
func (function) – a Python function that takes Tensor inputs and returns a Tensor or a tuple of Tensors
inputs (tuple of Tensor or Tensor) – inputs to the function
grad_outputs (tuple of Tensor or Tensor, optional) – The gradients with respect to the function’s outputs.
eps (float, optional) – perturbation for finite differences
atol (float, optional) – absolute tolerance
rtol (float, optional) – relative tolerance
gen_non_contig_grad_outputs (bool, optional) – if
grad_outputs
isNone
andgen_non_contig_grad_outputs
isTrue
, the randomly generated gradient outputs are made to be noncontiguousraise_exception (bool, optional) – indicating whether to raise an exception if the check fails. The exception gives more information about the exact nature of the failure. This is helpful when debugging gradchecks.
nondet_tol (float, optional) – tolerance for nondeterminism. When running identical inputs through the differentiation, the results must either match exactly (default, 0.0) or be within this tolerance. Note that a small amount of nondeterminism in the gradient will lead to larger inaccuracies in the second derivative.
 Returns
True if all differences satisfy allclose condition
Profiler¶
Autograd includes a profiler that lets you inspect the cost of different
operators inside your model  both on the CPU and GPU. There are two modes
implemented at the moment  CPUonly using profile
.
and nvprof based (registers both CPU and GPU activity) using
emit_nvtx
.

class
torch.autograd.profiler.
profile
(enabled=True, use_cuda=False, record_shapes=False)[source]¶ Context manager that manages autograd profiler state and holds a summary of results. Under the hood it just records events of functions being executed in C++ and exposes those events to Python. You can wrap any code into it and it will only report runtime of PyTorch functions.
 Parameters
enabled (bool, optional) – Setting this to False makes this context manager a noop. Default:
True
.use_cuda (bool, optional) – Enables timing of CUDA events as well using the cudaEvent API. Adds approximately 4us of overhead to each tensor operation. Default:
False
record_shapes (bool, optional) – If shapes recording is set, information about input dimensions will be collected. This allows one to see which dimensions have been used under the hood and further group by them using prof.key_averages(group_by_input_shape=True). Please note that shape recording might skew your profiling data. It is recommended to use separate runs with and without shape recording to validate the timing. Most likely the skew will be negligible for bottom most events (in a case of nested function calls). But for higher level functions the total self cpu time might be artificially increased because of the shape collection.
Example
>>> x = torch.randn((1, 1), requires_grad=True) >>> with torch.autograd.profiler.profile() as prof: >>> for _ in range(100): # any normal python code, really! >>> y = x ** 2 >> y.backward() >>> # NOTE: some columns were removed for brevity >>> print(prof.key_averages().table(sort_by="self_cpu_time_total"))     Name Self CPU total CPU time avg Number of Calls     mul 32.048ms 32.048ms 200 pow 27.041ms 27.041ms 200 PowBackward0 9.727ms 55.483ms 100 torch::autograd::AccumulateGrad 9.148ms 9.148ms 100 torch::autograd::GraphRoot 691.816us 691.816us 100    

export_chrome_trace
(path)[source]¶ Exports an EventList as a Chrome tracing tools file.
The checkpoint can be later loaded and inspected under
chrome://tracing
URL. Parameters
path (str) – Path where the trace will be written.

key_averages
(group_by_input_shape=False)[source]¶ Averages all function events over their keys.
@param group_by_input_shapes The key would become (event name, input dimensions) rather than just event name. This is useful to see which dimensionality contributes to the runtime the most and may help with dimension specific optimizations or choosing best candidates for quantization (aka fitting a roof line)
 Returns
An EventList containing FunctionEventAvg objects.

property
self_cpu_time_total
¶ Returns total time spent on CPU obtained as a sum of all self times across all the events.

table
(sort_by=None, row_limit=100, header=None)[source]¶ Prints an EventList as a nicely formatted table.
 Parameters
sort_by (str, optional) – Attribute used to sort entries. By default they are printed in the same order as they were registered. Valid keys include:
cpu_time
,cuda_time
,cpu_time_total
,cuda_time_total
,count
. Returns
A string containing the table.

class
torch.autograd.profiler.
record_function
(name)[source]¶ Context manager/function decorator that adds a label to a block of Python code (or function) when running autograd profiler. It is useful when tracing the code profile.
 Parameters
name (str) – Label assigned to the block of code.
Example
>>> x = torch.randn((1, 1), requires_grad=True) >>> with torch.autograd.profiler.profile() as prof: ... y = x ** 2 ... with torch.autograd.profiler.record_function("labelz"): # label the block ... z = y ** 3 ... y.backward() ... >>> # NOTE: some columns were removed for brevity >>> print(prof.key_averages().table(sort_by="self_cpu_time_total"))     Name Self CPU total % CPU time avg Number of Calls     pow 60.77% 47.470us 3 mul 21.73% 25.465us 2 PowBackward0 12.03% 121.891us 1 torch::autograd::AccumulateGrad 2.70% 6.324us 1 labelz 2.13% 12.421us 1 torch::autograd::GraphRoot 0.64% 1.503us 1     Self CPU time total: 234.344us CUDA time total: 0.000us

class
torch.autograd.profiler.
emit_nvtx
(enabled=True, record_shapes=False)[source]¶ Context manager that makes every autograd operation emit an NVTX range.
It is useful when running the program under nvprof:
nvprof profilefromstart off o trace_name.prof  <regular command here>
Unfortunately, there’s no way to force nvprof to flush the data it collected to disk, so for CUDA profiling one has to use this context manager to annotate nvprof traces and wait for the process to exit before inspecting them. Then, either NVIDIA Visual Profiler (nvvp) can be used to visualize the timeline, or
torch.autograd.profiler.load_nvprof()
can load the results for inspection e.g. in Python REPL. Parameters
enabled (bool, optional, default=True) – Setting
enabled=False
makes this context manager a noop. Default:True
.record_shapes (bool, optional, default=False) – If
record_shapes=True
, the nvtx range wrapping each autograd op will append information about the sizes of Tensor arguments received by that op, in the following format:[[arg0.size(0), arg0.size(1), ...], [arg1.size(0), arg1.size(1), ...], ...]
Nontensor arguments will be represented by[]
. Arguments will be listed in the order they are received by the backend op. Please note that this order may not match the order in which those arguments were passed on the Python side. Also note that shape recording may increase the overhead of nvtx range creation.
Example
>>> with torch.cuda.profiler.profile(): ... model(x) # Warmup CUDA memory allocator and profiler ... with torch.autograd.profiler.emit_nvtx(): ... model(x)
Forwardbackward correlation
When viewing a profile created using
emit_nvtx
in the Nvidia Visual Profiler, correlating each backwardpass op with the corresponding forwardpass op can be difficult. To ease this task,emit_nvtx
appends sequence number information to the ranges it generates.During the forward pass, each function range is decorated with
seq=<N>
.seq
is a running counter, incremented each time a new backward Function object is created and stashed for backward. Thus, theseq=<N>
annotation associated with each forward function range tells you that if a backward Function object is created by this forward function, the backward object will receive sequence number N. During the backward pass, the toplevel range wrapping each C++ backward Function’sapply()
call is decorated withstashed seq=<M>
.M
is the sequence number that the backward object was created with. By comparingstashed seq
numbers in backward withseq
numbers in forward, you can track down which forward op created each backward Function.Any functions executed during the backward pass are also decorated with
seq=<N>
. During default backward (withcreate_graph=False
) this information is irrelevant, and in fact,N
may simply be 0 for all such functions. Only the toplevel ranges associated with backward Function objects’apply()
methods are useful, as a way to correlate these Function objects with the earlier forward pass.Doublebackward
If, on the other hand, a backward pass with
create_graph=True
is underway (in other words, if you are setting up for a doublebackward), each function’s execution during backward is given a nonzero, usefulseq=<N>
. Those functions may themselves create Function objects to be executed later during doublebackward, just as the original functions in the forward pass did. The relationship between backward and doublebackward is conceptually the same as the relationship between forward and backward: The functions still emit currentsequencenumbertagged ranges, the Function objects they create still stash those sequence numbers, and during the eventual doublebackward, the Function objects’apply()
ranges are still tagged withstashed seq
numbers, which can be compared to seq numbers from the backward pass.
Anomaly detection¶

class
torch.autograd.
detect_anomaly
[source]¶ Contextmanager that enable anomaly detection for the autograd engine.
This does two things:  Running the forward pass with detection enabled will allow the backward pass to print the traceback of the forward operation that created the failing backward function.  Any backward computation that generate “nan” value will raise an error.
Warning
This mode should be enabled only for debugging as the different tests will slow down your program execution.
Example
>>> import torch >>> from torch import autograd >>> class MyFunc(autograd.Function): ... @staticmethod ... def forward(ctx, inp): ... return inp.clone() ... @staticmethod ... def backward(ctx, gO): ... # Error during the backward pass ... raise RuntimeError("Some error in backward") ... return gO.clone() >>> def run_fn(a): ... out = MyFunc.apply(a) ... return out.sum() >>> inp = torch.rand(10, 10, requires_grad=True) >>> out = run_fn(inp) >>> out.backward() Traceback (most recent call last): File "<stdin>", line 1, in <module> File "/your/pytorch/install/torch/tensor.py", line 93, in backward torch.autograd.backward(self, gradient, retain_graph, create_graph) File "/your/pytorch/install/torch/autograd/__init__.py", line 90, in backward allow_unreachable=True) # allow_unreachable flag File "/your/pytorch/install/torch/autograd/function.py", line 76, in apply return self._forward_cls.backward(self, *args) File "<stdin>", line 8, in backward RuntimeError: Some error in backward >>> with autograd.detect_anomaly(): ... inp = torch.rand(10, 10, requires_grad=True) ... out = run_fn(inp) ... out.backward() Traceback of forward call that caused the error: File "tmp.py", line 53, in <module> out = run_fn(inp) File "tmp.py", line 44, in run_fn out = MyFunc.apply(a) Traceback (most recent call last): File "<stdin>", line 4, in <module> File "/your/pytorch/install/torch/tensor.py", line 93, in backward torch.autograd.backward(self, gradient, retain_graph, create_graph) File "/your/pytorch/install/torch/autograd/__init__.py", line 90, in backward allow_unreachable=True) # allow_unreachable flag File "/your/pytorch/install/torch/autograd/function.py", line 76, in apply return self._forward_cls.backward(self, *args) File "<stdin>", line 8, in backward RuntimeError: Some error in backward

class
torch.autograd.
set_detect_anomaly
(mode)[source]¶ Contextmanager that sets the anomaly detection for the autograd engine on or off.
set_detect_anomaly
will enable or disable the autograd anomaly detection based on its argumentmode
. It can be used as a contextmanager or as a function.See
detect_anomaly
above for details of the anomaly detection behaviour. Parameters
mode (bool) – Flag whether to enable anomaly detection (
True
), or disable (False
).