Back out "Vectorize reduction when reducing on fastest striding dimension" (#36854)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/36854
Original commit changeset: ea3f7f29709c
Test Plan: n/a
Differential Revision: D21103684
fbshipit-source-id: e4862b32bf9815486e5fa7e05b9816550e9b0263
