Document reduce_scatter collective operation (#35274)
Summary:
I don't know why reduce_scatter collective operation is not documented so I add it to the document.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/35274
Differential Revision: D20645850
Pulled By: mrshenli
fbshipit-source-id: 0a4458bff1a4e15a4593dd4dcc25e4e0f6e2265d
