Clarify track_running_stats docs; Make SyncBatchNorm track_running_stats behavior consistent (#44445)
Summary:
context: https://github.com/pytorch/pytorch/pull/38084
Fixes #{issue number}
Pull Request resolved: https://github.com/pytorch/pytorch/pull/44445
Reviewed By: colesbury
Differential Revision: D23634216
Pulled By: mrshenli
fbshipit-source-id: d1242c694dec0e7794651f8031327625eb9989ee
