[pytorch] Change autograd fallback mode to Nothing (#105505)
Summary:
This caused some internal tests to fail. I'm not sure it is possible to easily
revert the original diff. This diff is a hotfix that changes the autograd
fallback behavior to what it was previously.
Test Plan: Existing tests
Differential Revision: D47569822
Pull Request resolved: https://github.com/pytorch/pytorch/pull/105505
Approved by: https://github.com/soulitzer
