Strided masked log_softmax. (#68461)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/68461
Test Plan: Imported from OSS
Reviewed By: dagitses, zou3519
Differential Revision: D32569961
Pulled By: cpuhrsch
fbshipit-source-id: 5d262adacf239dace4a28de85af4b602e36f17f0
