[DDP] Disable reducer hooks from running outside of DDP backwards. (#60921)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/60921
Sometimes local modules can fire hooks (such as when user calls
backward after using `ddp_module.module` explicitly). This isn't supported
behavior and can cause issues with various state and gradient reduction we run
in DDP, so it's best to disable this entirely.
ghstack-source-id: 132739311
Test Plan: CI
Reviewed By: SciPioneer
Differential Revision: D29435737
fbshipit-source-id: fef76a0dd2955c432131632fb81dde4a4982ad91
