[autograd.Function] Add docs on the functorch interaction (#91452)
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
