Add setup_context staticmethod to autograd.Function (#89859)
Adds a setup_context staticmethod to autograd.Function.
If it exists, then the user splits the ctx-specific logic from the
forward() and puts it in the setup_context staticmethod.
Docs will come later when we remove the feature flag.
Test Plan:
- some light tests
Pull Request resolved: https://github.com/pytorch/pytorch/pull/89859
Approved by: https://github.com/soulitzer
