Improve numerical stability of GroupNorm (#54921)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/54921
Improve numerical stability of GroupNorm
Test Plan: buck test mode/dev-nosan //caffe2/test:nn -- "GroupNorm"
Reviewed By: ngimel
Differential Revision: D27414438
fbshipit-source-id: 815517240ca5ea3e2beb77ced3bd862e9c83d445
