[metal] fix op registration for cat (#77451)
Summary: Registration for the concatenation op has seemingly changed from `aten::_cat` to `aten::cat`. This diff resolves the "op not found" error caused by the mismatched registration.
Test Plan:
Test in pytorch playground:
```
arc focus2 -b pp-ios -a ModelRunner -a //xplat/caffe2/c10:c10Apple -a //xplat/caffe2:torch_mobile_coreApple -a //xplat/caffe2/fb/dynamic_pytorch:dynamic_pytorch_implApple -a //xplat/caffe2:coreml_delegateApple -a ModelRunnerDevOps -a //xplat/caffe2:torch_mobile_all_opsApple -fd --force-with-wrong-xcode
```
Differential Revision: D36387105
Pull Request resolved: https://github.com/pytorch/pytorch/pull/77451
Approved by: https://github.com/axitkhurana, https://github.com/kimishpatel
