Add forward AD formulas for mv, scatter_add, _s_where (#70468)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/70468
Test Plan: Imported from OSS
Reviewed By: albanD
Differential Revision: D33405364
Pulled By: soulitzer
fbshipit-source-id: 7681c33fb264a7a3ec6436ebb7c5bb07cd5ffc3d
