Add new reduce options and autograd support for scatter_reduce (#71788)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/71788
Test Plan: Imported from OSS
Reviewed By: mikaylagawarecki
Differential Revision: D33778525
Pulled By: cpuhrsch
fbshipit-source-id: 47b8544e29df3075bc6ede894c59499a7ffec876
(cherry picked from commit ddcddac7262dd78a4002aaaea08daa3c50526028)
