Strided masked norm. (#68584)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/68584
Test Plan: Imported from OSS
Reviewed By: pbelevich
Differential Revision: D32581285
Pulled By: cpuhrsch
fbshipit-source-id: 896ee1e58957b46c2f6a16a170adff4cb3b8da62
