[autograd.Function] enable the extended Function feature flag by default (#91441)
The autograd.Function <> functorch interaction is in a mostly completed
state now. There are some minor action items remaining
(https://github.com/pytorch/pytorch/issues/90224), but I want to enable
the feature by default so that PyTorch CI / other parties / etc can
begin testing to see if there is any impact on the original
autograd.Function API (there shouldn't be).
The longer-term plan for the feature flag is:
- keep it around until at least the next release (so that people can
turn off the feature if it breaks something in existing code)
- delete the flag then (either before or after the release, I haven't
decided yet)
Test Plan:
- new test
- wait for CI
Pull Request resolved: https://github.com/pytorch/pytorch/pull/91441
Approved by: https://github.com/albanD, https://github.com/soulitzer
