Add elementwise_affine for LayerNormGradientOp (#19982)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/19982
Add elementwise_affine for LayerNormGradientOp
Reviewed By: houseroad
Differential Revision: D15157493
fbshipit-source-id: 7465f2c1d4df4649b4903b93483c4861e9c7afa9
