[Shape Fns] Fix handling of empty dim list in sum_mean_dim shape fn (#83357)
The current implementation of the `sum_mean_dim` shape function
takes `dim=[]` and `dim=None` to mean "no reduction". However, in the
ops `torch.sum` and `torch.mean`, both `dim=[]` and `dim=None` are
equivalent to "reduce along all dimensions". This commit fixes the
handling of `dim` in the `sum_mean_dim` shape function.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/83357
Approved by: https://github.com/Gamrix
