add SyncBatchNorm to docs (#18988)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/18983
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18988
Differential Revision: D14820042
Pulled By: soumith
fbshipit-source-id: 356169f554a42303b266d700d3379a5288f9671d
