Extending PyTorch
=================

In this note we'll cover ways of extending :mod:`torch.nn`,
:mod:`torch.autograd`, :mod:`torch`, and writing custom C extensions utilizing our C
libraries.

Extending :mod:`torch.autograd`
-------------------------------

.. currentmodule:: torch.autograd

Adding operations to :mod:`~torch.autograd` requires implementing a new
:class:`Function` subclass for each operation. Recall that :class:`Function` s
are what :mod:`~torch.autograd` uses to compute the results and gradients, and
encode the operation history. Every new function requires you to implement 2 methods:

- :meth:`~Function.forward` - the code that performs the operation. It can take
  as many arguments as you want, with some of them being optional, if you
  specify the default values. All kinds of Python objects are accepted here.
  :class:`Tensor` arguments that track history (i.e., with
  ``requires_grad=True``) will be converted to ones that don't track history
  before the call, and their use will be registered in the graph. Note that this
  logic won't traverse lists/dicts/any other data structures and will only
  consider :class:`Tensor` s that are direct arguments to the call. You can
  return either a single :class:`Tensor` output, or a :class:`tuple` of
  :class:`Tensor` s if there are multiple outputs. Also, please refer to the
  docs of :class:`Function` to find descriptions of useful methods that can be
  called only from :meth:`~Function.forward`.
- :meth:`~Function.backward` - gradient formula. It will be given
  as many :class:`Tensor` arguments as there were outputs, with each of them
  representing gradient w.r.t. that output. It should return as many
  :class:`Tensor` s as there were inputs, with each of them containing the
  gradient w.r.t. its corresponding input. If your inputs didn't require
  gradient (:attr:`~ctx.needs_input_grad` is a tuple of booleans indicating
  whether each input needs gradient computation), or were non-:class:`Tensor`
  objects, you can return :class:`python:None`. Also, if you have optional
  arguments to :meth:`~Function.forward` you can return more gradients than there
  were inputs, as long as they're all :any:`python:None`.

.. note::

  It's the user's responsibility to use the special functions in the forward's `ctx`
  properly in order to ensure that the new :class:`Function` works properly with
  the autograd engine.

  - :meth:`~torch.autograd.function._ContextMethodMixin.save_for_backward` must be
    used when saving input or output of the forward to be used later in the backward.
  - :meth:`~torch.autograd.function._ContextMethodMixin.mark_dirty` must be used to
    mark any input that is modified inplace by the forward function.
  - :meth:`~torch.autograd.function._ContextMethodMixin.mark_non_differentiable` must
    be used to tell the engine if an output is not differentiable.


Below you can find code for a ``Linear`` function from :mod:`torch.nn`, with
additional comments::

    # Inherit from Function
    class LinearFunction(Function):

        # Note that both forward and backward are @staticmethods
        @staticmethod
        # bias is an optional argument
        def forward(ctx, input, weight, bias=None):
            ctx.save_for_backward(input, weight, bias)
            output = input.mm(weight.t())
            if bias is not None:
                output += bias.unsqueeze(0).expand_as(output)
            return output

        # This function has only a single output, so it gets only one gradient
        @staticmethod
        def backward(ctx, grad_output):
            # This is a pattern that is very convenient - at the top of backward
            # unpack saved_tensors and initialize all gradients w.r.t. inputs to
            # None. Thanks to the fact that additional trailing Nones are
            # ignored, the return statement is simple even when the function has
            # optional inputs.
            input, weight, bias = ctx.saved_tensors
            grad_input = grad_weight = grad_bias = None

            # These needs_input_grad checks are optional and there only to
            # improve efficiency. If you want to make your code simpler, you can
            # skip them. Returning gradients for inputs that don't require it is
            # not an error.
            if ctx.needs_input_grad[0]:
                grad_input = grad_output.mm(weight)
            if ctx.needs_input_grad[1]:
                grad_weight = grad_output.t().mm(input)
            if bias is not None and ctx.needs_input_grad[2]:
                grad_bias = grad_output.sum(0)

            return grad_input, grad_weight, grad_bias

Now, to make it easier to use these custom ops, we recommend aliasing their
``apply`` method::

    linear = LinearFunction.apply

Here, we give an additional example of a function that is parametrized by
non-Tensor arguments::

    class MulConstant(Function):
        @staticmethod
        def forward(ctx, tensor, constant):
            # ctx is a context object that can be used to stash information
            # for backward computation
            ctx.constant = constant
            return tensor * constant

        @staticmethod
        def backward(ctx, grad_output):
            # We return as many input gradients as there were arguments.
            # Gradients of non-Tensor arguments to forward must be None.
            return grad_output * ctx.constant, None

.. note::
    Inputs to ``backward``, i.e., :attr:`grad_output`, can also be Tensors that
    track history. So if ``backward`` is implemented with differentiable
    operations, (e.g., invocation of another custom
    :class:`~torch.autograd.function`), higher order derivatives will work.

You probably want to check if the backward method you implemented actually
computes the derivatives of your function. It is possible by comparing with
numerical approximations using small finite differences::

    from torch.autograd import gradcheck

    # gradcheck takes a tuple of tensors as input, check if your gradient
    # evaluated with these tensors are close enough to numerical
    # approximations and returns True if they all verify this condition.
    input = (torch.randn(20,20,dtype=torch.double,requires_grad=True), torch.randn(30,20,dtype=torch.double,requires_grad=True))
    test = gradcheck(linear, input, eps=1e-6, atol=1e-4)
    print(test)

See :ref:`grad-check` for more details on finite-difference gradient comparisons.

Extending :mod:`torch.nn`
-------------------------

.. currentmodule:: torch.nn

:mod:`~torch.nn` exports two kinds of interfaces - modules and their functional
versions. You can extend it in both ways, but we recommend using modules for
all kinds of layers, that hold any parameters or buffers, and recommend using
a functional form parameter-less operations like activation functions, pooling,
etc.

Adding a functional version of an operation is already fully covered in the
section above.

Adding a :class:`Module`
^^^^^^^^^^^^^^^^^^^^^^^^

Since :mod:`~torch.nn` heavily utilizes :mod:`~torch.autograd`, adding a new
:class:`Module` requires implementing a :class:`~torch.autograd.Function`
that performs the operation and can compute the gradient. From now on let's
assume that we want to implement a ``Linear`` module and we have the function
implemented as in the listing above. There's very little code required to
add this. Now, there are two functions that need to be implemented:

- ``__init__`` (*optional*) - takes in arguments such as kernel sizes, numbers
  of features, etc. and initializes parameters and buffers.
- :meth:`~Module.forward` - instantiates a :class:`~torch.autograd.Function` and
  uses it to perform the operation. It's very similar to a functional wrapper
  shown above.

This is how a ``Linear`` module can be implemented::

    class Linear(nn.Module):
        def __init__(self, input_features, output_features, bias=True):
            super(Linear, self).__init__()
            self.input_features = input_features
            self.output_features = output_features

            # nn.Parameter is a special kind of Tensor, that will get
            # automatically registered as Module's parameter once it's assigned
            # as an attribute. Parameters and buffers need to be registered, or
            # they won't appear in .parameters() (doesn't apply to buffers), and
            # won't be converted when e.g. .cuda() is called. You can use
            # .register_buffer() to register buffers.
            # nn.Parameters require gradients by default.
            self.weight = nn.Parameter(torch.Tensor(output_features, input_features))
            if bias:
                self.bias = nn.Parameter(torch.Tensor(output_features))
            else:
                # You should always register all possible parameters, but the
                # optional ones can be None if you want.
                self.register_parameter('bias', None)

            # Not a very smart way to initialize weights
            self.weight.data.uniform_(-0.1, 0.1)
            if bias is not None:
                self.bias.data.uniform_(-0.1, 0.1)

        def forward(self, input):
            # See the autograd section for explanation of what happens here.
            return LinearFunction.apply(input, self.weight, self.bias)

        def extra_repr(self):
            # (Optional)Set the extra information about this module. You can test
            # it by printing an object of this class.
            return 'input_features={}, output_features={}, bias={}'.format(
                self.input_features, self.output_features, self.bias is not None
            )

Extending :mod:`torch`
----------------------

You can create custom types that emulate :class:`Tensor` by defining a custom
class with methods that match :class:`Tensor`. But what if you want to be able
to pass these types to functions like :func:`torch.add` in the top-level
:mod:`torch` namespace that accept :class:`Tensor` operands?

If your custom python type defines a method named ``__torch_function__``, PyTorch
will invoke your ``__torch_function__`` implementation when an instance of your
custom class is passed to a function in the :mod:`torch` namespace. This makes
it possible to define custom implementations for any of the functions in the
:mod:`torch` namespace which your ``__torch_function__`` implementation can call,
allowing your users to make use of your custom type with existing PyTorch
workflows that they have already written for :class:`Tensor`. This works with
"duck" types that are unrelated to :class:`Tensor` as well as user-defined
subclasses of :class:`Tensor`.

Extending :mod:`torch` with a :class:`Tensor`-like type
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

.. note:: This functionality is inspired by the NumPy ``__array_function__``
          protocol. See `the NumPy documentation
          <https://docs.scipy.org/doc/numpy/user/basics.dispatch.html#basics-dispatch>`_
          and `NEP-0018
          <https://numpy.org/neps/nep-0018-array-function-protocol.html>`_ for
          more details.

To make this concrete, let's begin with a simple example that illustrates the
API dispatch mechanism. We'll create a custom type that represents a 2D scalar
tensor, parametrized by the order ``N`` and value along the diagonal entries,
``value``::

     class ScalarTensor(object):
        def __init__(self, N, value):
            self._N = N
            self._value = value

        def __repr__(self):
            return "DiagonalTensor(N={}, value={})".format(self._N, self._value)

        def tensor(self):
            return self._value * torch.eye(self._N)

This first iteration of the design isn't very useful. The main functionality of
``ScalarTensor`` is to provide a more compact string representation of a scalar
tensor than in the base tensor class::

  >>> d = ScalarTensor(5, 2)
  >>> d
  ScalarTensor(N=5, value=2)
  >>> d.tensor()
  tensor([[2., 0., 0., 0., 0.],
          [0., 2., 0., 0., 0.],
          [0., 0., 2., 0., 0.],
          [0., 0., 0., 2., 0.],
          [0., 0., 0., 0., 2.]])

If we try to use this object with the :mod:`torch` API, we will run
into issues::

  >>> import torch
  >>> torch.mean(d)
  TypeError: mean(): argument 'input' (position 1) must be Tensor, not ScalarTensor

Adding a ``__torch_function__`` implementation to ``ScalarTensor`` makes it
possible for the above operation to succeed. Let's re-do our implementation,
this time adding a ``__torch_function__`` implementation::

  HANDLED_FUNCTIONS = {}
  class ScalarTensor(object):
      def __init__(self, N, value):
          self._N = N
          self._value = value

      def __repr__(self):
          return "DiagonalTensor(N={}, value={})".format(self._N, self._value)

      def tensor(self):
          return self._value * torch.eye(self._N)

      def __torch_function__(self, func, args=(), kwargs=None):
          if kwargs is None:
              kwargs = {}
          if func not in HANDLED_FUNCTIONS:
              return NotImplemented
          return HANDLED_FUNCTIONS[func](*args, **kwargs)

The ``__torch_function__`` method takes three arguments: ``func``, a reference to
the torch API function that is being overrided, ``args``, the tuple of arguments
passed to the function, and ``kwargs``, the dict of keyword arguments passed to
the function. It uses a global dispatch stable named ``HANDLED_FUNCTIONS`` to
store custom implementations. The keys of this dictionary are functions in the
``torch`` namespace and the values are implementations for ``ScalarTensor``.

.. note:: Using a global dispatch table is not a mandated part of the
          ``__torch_function__`` API, it is just a useful design pattern for
          structuring your override implementations.

This class definition isn't quite enough to make ``torch.mean`` do the right
thing when we pass it a ``ScalarTensor`` -- we also need to define an
implementation for ``torch.mean`` for ``ScalarTensor`` operands and add the
implementation to the ``HANDLED_FUNCTIONS`` dispatch table dictionary. One way
of doing this is to define a decorator::

  import functools
  def implements(torch_function):
      """Register a torch function override for ScalarTensor"""
      @functools.wraps(torch_function)
      def decorator(func):
          HANDLED_FUNCTIONS[torch_function] = func
          return func
      return decorator

which can be applied to the implementation of our override::

  @implements(torch.mean)
  def mean(input):
      return float(input._value) / input._N

With this change we can now use ``torch.mean`` with ``ScalarTensor``::

  >>> d = ScalarTensor(5, 2)
  >>> torch.mean(d)
  0.4

Of course ``torch.mean`` is an example of the simplest kind of function to
override since it only takes one operand. We can use the same machinery to
override a function that takes more than one operand, any one of which might be
a tensor or tensor-like that defines ``__torch_function__``, for example for
:func:`torch.add`::

  def ensure_tensor(data):
      if isinstance(data, ScalarTensor):
          return data.tensor()
      return torch.as_tensor(data)

  @implements(torch.add)
  def add(input, other):
     try:
         if input._N == other._N:
             return ScalarTensor(input._N, input._value + other._value)
         else:
             raise ValueError("Shape mismatch!")
     except AttributeError:
         return torch.add(ensure_tensor(input), ensure_tensor(other))

This version has a fast path for when both operands are ``ScalarTensor``
instances and also a slower path which degrades to converting the data to
tensors when either operand is not a ``ScalarTensor``. That makes the override
function correctly when either operand is a ``ScalarTensor`` or a regular
:class:`Tensor`::

  >>> s = ScalarTensor(2, 2)
  >>> torch.add(s, s)
  DiagonalTensor(N=2, value=4)
  >>> t = torch.tensor([[1, 1,], [1, 1]])
  >>> torch.add(s, t)
  tensor([[3., 1.],
          [1., 3.]])

Note that our implementation of ``add`` does not take ``alpha`` or ``out`` as
keyword arguments like :func:`torch.add` does::

  >>> torch.add(s, s, alpha=2)
  TypeError: add() got an unexpected keyword argument 'alpha'

For speed and flexibility the ``__torch_function__`` dispatch mechanism does not
check that the signature of an override function matches the signature of the
function being overrided in the :mod:`torch` API. For some applications ignoring
optional arguments would be fine but to ensure full compatibility with
:class:`Tensor`, user implementations of torch API functions should take care to
exactly emulate the API of the function that is being overrided.

Functions in the :mod:`torch` API that do not have explicit overrides will
return ``NotImplemented`` from ``__torch_function__``. If all operands with
``__torch_function__`` defined on them return ``NotImplemented``, PyTorch will
raise a ``TypeError``. This means that most of the time operations that do not
have explicit overrides for a type will raise a ``TypeError`` when an instance
of such a type is passed::

  >>> torch.mul(s, 3)
  TypeError: no implementation found for 'torch.mul' on types that
  implement __torch_function__: [ScalarTensor]

In practice this means that if you would like to implement your overrides using
a ``__torch_function__`` implementation along these lines, you will need to
explicitly implement the full :mod:`torch` API or the entire subset of the API
that you care about for your use case. This may be a tall order as the full
:mod:`torch` API is quite extensive.

Another option is to not return ``NotImplemented`` for operations that are not
handled but to instead pass a :class:`Tensor` to the original :mod:`torch`
function when no override is available. For example, if we change our
implementation of ``__torch_function__`` for ``ScalarTensor`` to the one below::

  def __torch_function__(self, func, args=(), kwargs=None):
      if kwargs is None:
          kwargs = {}
      if func not in HANDLED_FUNCTIONS:
          args = [a.tensor() if hasattr(a, 'tensor') else a for a in args]
          return func(*args, **kwargs)
      return HANDLED_FUNCTIONS[func](*args, **kwargs)

Then :func:`torch.mul` will work correctly, although the return type will always
be a :class:`Tensor` rather than a :class:`ScalarTensor`, even if both operands
are :class:`ScalarTensor` instances::

  >>> s = ScalarTensor(2, 2)
  >>> torch.mul(s, s)
  tensor([[4., 0.],
          [0., 4.]])

Also see the ``MetadataTensor`` example below for another variation on this
pattern but instead always returns a ``MetadataTensor`` to propagate metadata
through operations in the :mod:`torch` API.

Extending :mod:`torch` with a :class:`Tensor` wrapper type
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Another useful case is a type that wraps a :class:`Tensor`, either as an
attribute or via subclassing. Below we implement a special case of this sort of
type, a ``MetadataTensor`` that attaches a dictionary of metadata to a
:class:`Tensor` that is propagated through :mod:`torch` operations. Since this
is a generic sort of wrapping for the full :mod:`torch` API, we do not need to
individually implement each override so we can make the ``__torch_function__``
implementation more permissive about what operations are allowed::

  class MetadataTensor(object):
      def __init__(self, data, metadata=None, **kwargs):
          self._t = torch.as_tensor(data, **kwargs)
          self._metadata = metadata

      def __repr__(self):
          return "Metadata:\n{}\n\ndata:\n{}".format(self._metadata, self._t)

      def __torch_function__(self, func, args=(), kwargs=None):
          if kwargs is None:
              kwargs = {}
          args = [a._t if hasattr(a, '_t') else a for a in args]
          ret = func(*args, **kwargs)
          return MetadataTensor(ret, metadata=self._metadata)

This simple implementation won't necessarily work with every function in the
:mod:`torch` API but it is good enough to capture most common operations::

  >>> metadata = {'owner': 'Ministry of Silly Walks'}
  >>> m = MetadataTensor([[1, 2], [3, 4]], metadata=metadata)
  >>> t = torch.tensor([[1, 2], [1, 2]]])
  >>> torch.add(t, m)
  Metadata:
  {'owner': 'Ministry of Silly Walks'}

  data:
  tensor([[2, 4],
          [4, 6]])
  >>> torch.mul(t, m)
  Metadata:
  {'owner': 'Ministry of Silly Walks'}

  data:
  tensor([[1, 4],
          [3, 8]])

Operations on multiple types that define ``__torch_function__``
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

It is possible to use the torch API with multiple distinct types that each have
a ``__torch_function__`` implementation, but special care must be taken. In such
a case the rules are:

* The dispatch operation gathers all distinct implementations of
  ``__torch_function__`` for each operand and calls them in order: subclasses
  before superclasses, and otherwise left to right in the operator expression.
* If any value other than ``NotImplemented`` is returned, that value is
  returned as the result. Implementations can register that they do not
  implement an operation by returning ``NotImplemented``.
* If all of the ``__torch_function__`` implementations return
  ``NotImplemented``, PyTorch raises a ``TypeError``.

Writing custom C++ extensions
-----------------------------

See this
`PyTorch tutorial <https://pytorch.org/tutorials/advanced/cpp_extension.html>`_
for a detailed explanation and examples.

Documentations are available at :doc:`../cpp_extension`.


Writing custom C extensions
---------------------------

Example available at
`this GitHub repository <https://github.com/pytorch/extension-ffi>`_.
