[iOS GPU][BE][1/n] Rename MPSCNNContext to MetalContext (#60280)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/60280
No significant changes besides renaming the class. In the future, we'll convert this objc class to c++.
ghstack-source-id: 131827490
Test Plan:
- CircleCI
- buck test pp-mac
Reviewed By: SS-JIA
Differential Revision: D29231824
fbshipit-source-id: a0d1327a55a0414011c78a7144d3b05f1579cf42
