[vmap] Prepend "legacy" to files for old vmap implementation (#90324)
We have an older torch.vmap implementation. It is no longer supported.
It still needs to exist somewhere for the sake of BC with
torch.autograd.functional.
This PR makes it clear what files are meant for implementing the old
vmap implementation. I've seen a couple of PRs recently adding support
for the old vmap implementation, so this will lessen the confusion.
Test Plan:
- CI
Pull Request resolved: https://github.com/pytorch/pytorch/pull/90324
Approved by: https://github.com/samdow
