[MPS] Add `group_norm[fwd+backward]` and `mean_var` (take 2) (#91190)
Use Prims to implement group_norm, group_norm_backward and mean_var
Use `torch._ops.ops` instead of `torch.ops` in numerous subpackages in
order to be able to make them importable from `torch/backend/mps/__init__.py` as this alias is defined in
https://github.com/pytorch/pytorch/blob/15af4b1ceea3b689354ad664675b930486804c8e/torch/__init__.py#L1095
is executed last during init process.
Add `__all__` to `torch/backends/mps/__init__.py` as well as alias all imports as private
Add `TestNNMPS.test_group_norm_backward` that validates no NaNs are generated during the backward pass
Fixes https://github.com/pytorch/pytorch/issues/88331
Pull Request resolved: https://github.com/pytorch/pytorch/pull/91190
Approved by: https://github.com/albanD
