[autograd.Function] setup_context always appears on the Function (#92312)
Previously, we used the existence of setup_context to switch between if
forward should take a ctx object or not.
To be consistent with all other staticmethod (which always exist on the
autograd.Function), this PR change it so that we use IF setup_context
gets overriden by the user to switch between if forward should take a
ctx object or not.
Fixes https://github.com/pytorch/pytorch/issues/91451
Test Plan:
- existing tests
Pull Request resolved: https://github.com/pytorch/pytorch/pull/92312
Approved by: https://github.com/albanD, https://github.com/soulitzer
