[vmap] Fix index_select support when dim is negative (#97916) (#98019)
Fixes https://github.com/pytorch/pytorch/issues/96854
Previously, this would segfault (via indexing -2 into a SmallVector).
This PR fixes it so that we wrap negative dimensions.
Test Plan:
- changed the index_select OpInfo to use dim=-1 instead of dim=1,
because it's much more common that the negative dimension doesn't work
instead of the positive one.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/97916
Approved by: https://github.com/ngimel, https://github.com/janeyx99
