Strided masked softmin. (#68463)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/68463
Test Plan: Imported from OSS
Reviewed By: dagitses
Differential Revision: D32576497
Pulled By: cpuhrsch
fbshipit-source-id: 286edb2e7a5415df76858c69d0312743437b0fd8
