remove special grad_mode tls handling (#63116)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/63116
This PR removes the special flag to disable grad mode tracking on the ThreadLocalState and replaces it with an explicit setter that users can use.
This allows to reduce complexity of ThreadLocalState.
Test Plan: Imported from OSS
Reviewed By: ngimel
Differential Revision: D30388098
Pulled By: albanD
fbshipit-source-id: 85641b3d711179fb78ff6a41ed077548dc821a2f
