MPS: Implement aten::count_nonzero.dim_IntList (#78169)
- See: #77764
Implements the `aten::count_nonzero.dim_IntList` operator (as used by [torch.count_nonzero](https://pytorch.org/docs/stable/generated/torch.count_nonzero.html)) for [MPS](https://pytorch.org/blog/introducing-accelerated-pytorch-training-on-mac/).
Pull Request resolved: https://github.com/pytorch/pytorch/pull/78169
Approved by: https://github.com/malfet, https://github.com/kulinseth, https://github.com/albanD
