[autograd.Function] Cleanup asymmetry in generate_vmap_rule and vmap (#91787)
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
