Allow SyncBatchNorm without DDP in inference mode (#24815)
Summary:
Fix https://github.com/pytorch/pytorch/issues/22538
Pull Request resolved: https://github.com/pytorch/pytorch/pull/24815
Test Plan:
Can run a detectron2 evaluation without entering DDP.
#sandcastle
Differential Revision: D16883694
Pulled By: ppwwyyxx
fbshipit-source-id: 3195bc4e7f43a994821069f229b26302e2988739
