[torch.func] alias torch.func.vmap as torch.vmap (#91026)
This PR also redirects torch.vmap to torch.func.vmap instead of the old
vmap prototype.
Test Plan:
- tests
- view docs preview
Pull Request resolved: https://github.com/pytorch/pytorch/pull/91026
Approved by: https://github.com/albanD, https://github.com/samdow
