`scatter_reduce` documentation (#73125)
Summary:
Reland of https://github.com/pytorch/pytorch/issues/68580 (which were milestoned for 1.11) plus partial revert of https://github.com/pytorch/pytorch/pull/72543
Pull Request resolved: https://github.com/pytorch/pytorch/pull/73125
Reviewed By: bdhirsh
Differential Revision: D34355217
Pulled By: malfet
fbshipit-source-id: 325ecdeaf53183d653b44ee5e6e8839ceefd9200
(cherry picked from commit 71db31748a8adcd8f95d5faf04aaa454e9c4c760)
