[functorch] refactor get_fallback_and_vmap_exhaustive (#82897)
This PR refactors get_fallback_and_vmap_exhaustive into two helper
functions, `generate_vmap_inputs` and
`compute_quantities_for_vmap_test`. The former is responsible for
generating in_dims and the batched inputs; the latter is responsible for
computing vmap(blah) and the fallback.
The rationale for the refactor is that I want to use these pieces
separately in in-place vmap testing, which is coming soon.
Test Plan:
- run tests
Pull Request resolved: https://github.com/pytorch/pytorch/pull/82897
Approved by: https://github.com/Chillee
