[Docs] Add examples for new object-based c10d APIs (#43932)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/43932
Adds some basic examples to the documentation for each of the newly added
object-based collectibves.
ghstack-source-id: 117965966
Test Plan: CI
Reviewed By: mrshenli
Differential Revision: D23441838
fbshipit-source-id: 91344612952cfcaa71f08ccf2a2c9ed162ca9c89
