keep reuqires_grad unchanged after converting bn to syncbn (#22569)
Summary:
After converting BN layers to SyncBN layers, the function will set all `requires_grad = True` regardless of the original requires_grad states. I think it is a bug and have fixed it in this PR.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/22569
Differential Revision: D16151647
Pulled By: zou3519
fbshipit-source-id: e2ad1886c94d8882485e7fb8be51ad76469ecc67
