torch.func Whirlwind Tour
=========================
What is torch.func?
-------------------
.. currentmodule:: torch.func
torch.func, previously known as functorch, is a library for
`JAX <https://github.com/google/jax>`_-like composable function transforms in
PyTorch.
- A "function transform" is a higher-order function that accepts a numerical
function and returns a new function that computes a different quantity.
- torch.func has auto-differentiation transforms (``grad(f)`` returns a function
that computes the gradient of ``f``), a vectorization/batching transform
(``vmap(f)`` returns a function that computes ``f`` over batches of inputs),
and others.
- These function transforms can compose with each other arbitrarily. For
example, composing ``vmap(grad(f))`` computes a quantity called
per-sample-gradients that stock PyTorch cannot efficiently compute today.
Why composable function transforms?
-----------------------------------
There are a number of use cases that are tricky to do in PyTorch today:
- computing per-sample-gradients (or other per-sample quantities)
- running ensembles of models on a single machine
- efficiently batching together tasks in the inner-loop of MAML
- efficiently computing Jacobians and Hessians
- efficiently computing batched Jacobians and Hessians
Composing :func:`vmap`, :func:`grad`, :func:`vjp`, and :func:`jvp` transforms
allows us to express the above without designing a separate subsystem for each.
What are the transforms?
------------------------
:func:`grad` (gradient computation)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
``grad(func)`` is our gradient computation transform. It returns a new function
that computes the gradients of ``func``. It assumes ``func`` returns a single-element
Tensor and by default it computes the gradients of the output of ``func`` w.r.t.
to the first input.
.. code-block:: python
import torch
from torch.func import grad
x = torch.randn([])
cos_x = grad(lambda x: torch.sin(x))(x)
assert torch.allclose(cos_x, x.cos())
# Second-order gradients
neg_sin_x = grad(grad(lambda x: torch.sin(x)))(x)
assert torch.allclose(neg_sin_x, -x.sin())
:func:`vmap` (auto-vectorization)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Note: :func:`vmap` imposes restrictions on the code that it can be used on. For more
details, please see :ref:`ux-limitations`.
``vmap(func)(*inputs)`` is a transform that adds a dimension to all Tensor
operations in ``func``. ``vmap(func)`` returns a new function that maps ``func``
over some dimension (default: 0) of each Tensor in inputs.
vmap is useful for hiding batch dimensions: one can write a function func that
runs on examples and then lift it to a function that can take batches of
examples with ``vmap(func)``, leading to a simpler modeling experience:
.. code-block:: python
import torch
from torch.func import vmap
batch_size, feature_size = 3, 5
weights = torch.randn(feature_size, requires_grad=True)
def model(feature_vec):
# Very simple linear model with activation
assert feature_vec.dim() == 1
return feature_vec.dot(weights).relu()
examples = torch.randn(batch_size, feature_size)
result = vmap(model)(examples)
When composed with :func:`grad`, :func:`vmap` can be used to compute per-sample-gradients:
.. code-block:: python
from torch.func import vmap
batch_size, feature_size = 3, 5
def model(weights,feature_vec):
# Very simple linear model with activation
assert feature_vec.dim() == 1
return feature_vec.dot(weights).relu()
def compute_loss(weights, example, target):
y = model(weights, example)
return ((y - target) ** 2).mean() # MSELoss
weights = torch.randn(feature_size, requires_grad=True)
examples = torch.randn(batch_size, feature_size)
targets = torch.randn(batch_size)
inputs = (weights,examples, targets)
grad_weight_per_example = vmap(grad(compute_loss), in_dims=(None, 0, 0))(*inputs)
:func:`vjp` (vector-Jacobian product)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
The :func:`vjp` transform applies ``func`` to ``inputs`` and returns a new function
that computes the vector-Jacobian product (vjp) given some ``cotangents`` Tensors.
.. code-block:: python
from torch.func import vjp
inputs = torch.randn(3)
func = torch.sin
cotangents = (torch.randn(3),)
outputs, vjp_fn = vjp(func, inputs); vjps = vjp_fn(*cotangents)
:func:`jvp` (Jacobian-vector product)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
The :func:`jvp` transforms computes Jacobian-vector-products and is also known as
"forward-mode AD". It is not a higher-order function unlike most other transforms,
but it returns the outputs of ``func(inputs)`` as well as the jvps.
.. code-block:: python
from torch.func import jvp
x = torch.randn(5)
y = torch.randn(5)
f = lambda x, y: (x * y)
_, out_tangent = jvp(f, (x, y), (torch.ones(5), torch.ones(5)))
assert torch.allclose(out_tangent, x + y)
:func:`jacrev`, :func:`jacfwd`, and :func:`hessian`
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
The :func:`jacrev` transform returns a new function that takes in ``x`` and returns
the Jacobian of the function with respect to ``x`` using reverse-mode AD.
.. code-block:: python
from torch.func import jacrev
x = torch.randn(5)
jacobian = jacrev(torch.sin)(x)
expected = torch.diag(torch.cos(x))
assert torch.allclose(jacobian, expected)
:func:`jacrev` can be composed with :func:`vmap` to produce batched jacobians:
.. code-block:: python
x = torch.randn(64, 5)
jacobian = vmap(jacrev(torch.sin))(x)
assert jacobian.shape == (64, 5, 5)
:func:`jacfwd` is a drop-in replacement for jacrev that computes Jacobians using
forward-mode AD:
.. code-block:: python
from torch.func import jacfwd
x = torch.randn(5)
jacobian = jacfwd(torch.sin)(x)
expected = torch.diag(torch.cos(x))
assert torch.allclose(jacobian, expected)
Composing :func:`jacrev` with itself or :func:`jacfwd` can produce hessians:
.. code-block:: python
def f(x):
return x.sin().sum()
x = torch.randn(5)
hessian0 = jacrev(jacrev(f))(x)
hessian1 = jacfwd(jacrev(f))(x)
:func:`hessian` is a convenience function that combines jacfwd and jacrev:
.. code-block:: python
from torch.func import hessian
def f(x):
return x.sin().sum()
x = torch.randn(5)
hess = hessian(f)(x)