Add batched_grad parameter to `autograd.grad` (#65564)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/65564
- wrap the call into engine with vmap if `batched_grad` is `True`
- improves the comment on the call to engine (somewhat addressing https://github.com/pytorch/pytorch/issues/41659)
- borrows the message from functional.jacobian's vectorized argument concerning usage of the vmap feature
- adds basic test (further testing is done when we replace the usage in vectorized jacobian computation)
TODO:
- create an issue tracking this
Test Plan: Imported from OSS
Reviewed By: albanD
Differential Revision: D31236259
Pulled By: soulitzer
fbshipit-source-id: b33e6b26ea98fa9f70c44da08458fc54ba4df0f7
