Improve numerical stability of LayerNorm (#59987)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/59987
Similar as GroupNorm, improve numerical stability of LayerNorm by Welford algorithm and pairwise sum.
Test Plan: buck test mode/dev-nosan //caffe2/test:nn -- "LayerNorm"
Reviewed By: ngimel
Differential Revision: D29115235
fbshipit-source-id: 5183346c3c535f809ec7d98b8bdf6d8914bfe790
