Strided masked reduction: mean (2nd try) (#67088)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/67088
Stack from [ghstack](https://github.com/ezyang/ghstack):
* __->__ #67088
Test Plan: Imported from OSS
Reviewed By: anjali411
Differential Revision: D32070264
Pulled By: cpuhrsch
fbshipit-source-id: 08a91550dd24fb0f51abf06591a0e26186c4f9f9
