Fixes https://github.com/pytorch/pytorch/issues/97260
We got some feedback that the page reads like "in order to save an input
for backward, you must return it as an output of the
autograd.Function.forward".
Doing so actually raises an error (on master and as of 2.1), but results
in an ambiguous situation on 2.0.0. To avoid more users running into
this, we clarify the documentation so it doesn't read like the above
and clearly mentions that you can save things from the inputs or
outputs.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/98020
Approved by: https://github.com/soulitzer, https://github.com/kshitij12345
This PR:
- changes generate_vmap_rule to either be True or False. Previously it
could be True, False, or not set. This simplifies the implementation a
bit.
- changes the vmap staticmethod to always be on the autograd.Function
rather than sometimes defined.
This is how the other staticmethod (forward, backward, jvp) are
implemented and allows us to document it.
There are 4 possible states for the autograd.Function w.r.t. to the
above:
- generate_vmap_rule is True, vmap staticmethod overriden. This raises
an error when used with vmap.
- generate_vmap_rule is False, vmap staticmethod overriden. This is
valid.
- generate_vmap_rule is True, vmap staticmethod not overriden. This is
valid.
- generate_vmap_rule is False, vmap staticmethod not overriden. This
raises an error when used with vmap.
Future:
- setup_context needs the same treatment, but that's a bit tricker to
implement.
Test Plan:
- new unittest
- existing tests
Pull Request resolved: https://github.com/pytorch/pytorch/pull/91787
Approved by: https://github.com/soulitzer
This PR:
- Updates autograd.Function.forward docs to reflect how you either
define a forward with ctx or a separate forward and setup_context
- Updates the "Extending Autograd" docs to suggest the usage of
autograd.Function with separate forward and setup_context. This should
be the default because there is a low barrier to go from this to
an autograd.Function that is fully supported by functorch transforms.
- Adds a new "Extending torch.func with autograd.Function" doc that
explains how to use autograd.Function with torch.func. It also
explains how to use generate_vmap_rule and how to manually write a
vmap staticmethod.
While writing this, I noticed that the implementation of
setup_context staticmethod/generate_vmap_rule/vmap staticmethod are a
bit inconsistent with the other method/attributes on autograd.Function:
- https://github.com/pytorch/pytorch/issues/91451
- I'm happy to fix those if we think it is a problem, either in this PR
or a followup (this PR is getting long, I want some initial docs
out that I can point early adopters at, and fixing the problems in the
future isn't really BC-breaking).
Test Plan:
- view docs preview
Pull Request resolved: https://github.com/pytorch/pytorch/pull/91452
Approved by: https://github.com/soulitzer
