[Pytorch] Speed up LayerNorm 4-5% (#71423)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/71423
Replacing this math with a load seems to improve perf.
ghstack-source-id: 147171800
Test Plan: ptvsc2_predictor_bench runs on model from mikeiovine courtesy of mikeiovine
Reviewed By: mikeiovine, xiaomengy
Differential Revision: D33552176
fbshipit-source-id: f21a4cd66c13b9fcb7bcf48f356bdc85e94c4216
(cherry picked from commit 0354fcb9889e7345321fe4dc9e30495a67709a4d)
