[fix] legacybatching: getPhysicalDims (#93261)
Fixes #92985
Minimum Repro:
```python
import torch
from torch._vmap_internals import vmap
input = torch.randn(2, 2)
def fn(x):
return x.sum(())
o = vmap(fn)(input)
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/93261
Approved by: https://github.com/albanD, https://github.com/Skylion007
