disallow empty named dims list to flatten(names, name) (#61953)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/61137 by raising an error if an empty tuple is passed in for the names:
```
>>> torch.empty((2, 3), names=['a', 'b']).flatten((), 'abc')
RuntimeError: flatten(tensor, dims, out_dim): dims cannot be empty
```
or from the original issue:
```
>>> torch.empty((2, 3)).flatten((), 'abc')
RuntimeError: flatten(tensor, dims, out_dim): dims cannot be empty
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/61953
Reviewed By: iramazanli
Differential Revision: D30574571
Pulled By: malfet
fbshipit-source-id: e606e84458a8dd66e5da6d0eb1a260f37b4ce91b
