Add elementwise_affine for layer_norm_op (#19713)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/19713
Add elementwise_affine for layer_norm_op
Reviewed By: houseroad
Differential Revision: D15075454
fbshipit-source-id: e8a7d3da1c81e49fa55323f5e74a68bc4ef8d83f
