convert_sync_batch_norm to SyncBatchNorm (#18787)
Summary:
Closes #18382
Please let me know if any changes are required.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18787
Differential Revision: D14821147
Pulled By: soumith
fbshipit-source-id: edd98eab1b3f4151c4ae5148146435ddb2ae678d
