Revert D31914868: Strided masked reduction: mean (2nd try)
Test Plan: revert-hammer
Differential Revision:
D31914868 (https://github.com/pytorch/pytorch/commit/a33d3d84dfcaef93c17d78cf7983d62ee01d28e3)
Original commit changeset: beda9d32ea65
fbshipit-source-id: dc3fa66d7e3c8a211fedac6ae191b11a4a9ab232
