Optimize pytorch layer_norm forward (#20345)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/20345
Seperate from D15194600
Optimize pytorch layer_norm op part 1:
optimize layer_norm_forward_cpu
import Eigen Maps for the performance of reduction
Reviewed By: zheng-xq
Differential Revision: D15290608
fbshipit-source-id: cf2c208dfd6fbcbc4c69db3ed60278d9bee156b5
