Masked normalization infrastructure and strided masked softmax (#68333)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/68333
Test Plan: Imported from OSS
Reviewed By: dagitses, ZolotukhinM
Differential Revision: D32564435
Pulled By: cpuhrsch
fbshipit-source-id: 4d4662323ceffd12c210b7e931682d0442578157
