Add note about improved vmap prototype to vmap docstring (#57677)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/57677
This PR adds a note about the existence of the improved vmap prototype
to raise awareness of its existence. Eventually the plan is to delete
the in-core vmap prototype and replace it with the improved vmap
prototype but that might take a while.
Test Plan: - view docs
Reviewed By: Chillee
Differential Revision: D28231346
Pulled By: zou3519
fbshipit-source-id: 0a3b274df87ffd50333330e413e1a89634865403
