Beef up vmap docs and expose to master documentation (#44825)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/44825
Test Plan: - build and view docs locally.
Reviewed By: ezyang
Differential Revision: D23742727
Pulled By: zou3519
fbshipit-source-id: f62b7a76b5505d3387b7816c514c086c01089de0
