Strided masked reduction: mean (2nd try) (#67088)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/67088
Test Plan: Imported from OSS
Reviewed By: pbelevich
Differential Revision: D31914868
Pulled By: cpuhrsch
fbshipit-source-id: beda9d32ea65bcae31c2c0181f95ad23c6631075
