Move error messages in-line in `_vmap_internals.py` (#41077)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/41077
This PR is a refactor that moves error messages into their callsites in
`_vmap_internals.py`. Furthermore, because a little bird told me we've
dropped python 3.5 support, this PR adopts f-string syntax to clean up
the string replace logic. Together these changes make the error messages
read better IMO.
Test Plan:
- `python test/test_vmap.py -v`. There exists tests that invoke each of the
error messages.
Differential Revision: D22420473
Pulled By: zou3519
fbshipit-source-id: cfd46b2141ac96f0a62864928a95f8eaa3052f4e
