Initial vmap docstring (#40575)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/40575
This provides some more context for the next ~2 PRs that will implement
the `out_dims` and `in_dims` functionality. I will probably add more to
it later (things I think we should add: examples (maybe in a dedicated
docs page), specific examples of things vmap cannot handle).
Test Plan:
- Code reading for now. When we are ready to add vmap to master documentation,
I'll build the docs and fix any formatting problems.
Differential Revision: D22288085
Pulled By: zou3519
fbshipit-source-id: 6e28d7bd524242395160c20270159b4b121d6789
