Modify unique to return sizes when dim is zero-length
Fixes #75730
Previously, `torch.unique` returned a tensor of size `[0]` when the dimension supplied to `dim` is zero length. Instead, I've changed `torch.unique` to return a tensor matching the input size.
Changes:
- Modify `torch.unique` to return an an empty tensor of size `sizes` as the values output, when `dim` is zero length in the input tensor.
- Make the same change in the CUDA implementation of `torch.unique`.
- Update `test_unique_dim` to expect an empty tensor with size matching the input tensor.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/75764
Approved by: https://github.com/mruberry
