[autograd.Function] Kill the extension feature flag (#92026)
This PR removes the autograd.Function extension feature flag. This was
previously used for development of the functorch <> autograd.Function
interaction.
It's been in master for long enough with the feature flag defaulting to
True, so it's time to remove it.
Test Plan:
- existing tests
Pull Request resolved: https://github.com/pytorch/pytorch/pull/92026
Approved by: https://github.com/soulitzer
