Rename _scatter_reduce to scatter_reduce and make it unstructured (#71787)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/71787
Test Plan: Imported from OSS
Reviewed By: mikaylagawarecki
Differential Revision: D33778524
Pulled By: cpuhrsch
fbshipit-source-id: 55a330e1c2227c0eaaa1c0d2f9205a4dee24a11b
(cherry picked from commit 6e4a8a91dac179f2302f87964090a4f991a4392f)
