Rename flag that enables/disables _SingleLevelFunction for functorch (#92025)
functorch used to have a switch that enables/disables autograd.Function.
That switch now enables/disables torch.autograd.function._SingleLevelFunction, so
I've renamed it accordingly.
We could just delete the switch because users should not be directly
working with torch.autograd.function._SingleLevelFunction. However,
it was useful for debugging when something went wrong when I was
implementing the autograd.Function <> functorch interaction, so I want
to keep it around as a debugging tool for a while since the code is
already there.
Test Plan:
- updated tests
Pull Request resolved: https://github.com/pytorch/pytorch/pull/92025
Approved by: https://github.com/soulitzer
