add BFloat16 support for LayerNorm CPU (#55210)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/55210
Test Plan: Imported from OSS
Reviewed By: anjali411
Differential Revision: D28836793
Pulled By: VitalyFedyunin
fbshipit-source-id: 998298deedd7a18e45fb761a0a4e0d88b65f2e0c
