[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

test/test_legacy_vmap.py

@@ -3,7 +3,8 @@
 from torch.testing._internal.common_utils import TestCase, run_tests
 import torch
 import torch.nn.functional as F
-from torch import Tensor, vmap
+from torch import Tensor
+from torch._vmap_internals import vmap
 import functools
 import itertools
 import warnings