[iOS GPU] Move the definition of `fp16_t` to MetalUtils.h (#54521)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/54521
Move code around. No significant feature changes.
ghstack-source-id: 126802126
Test Plan:
- CircleCI
- Sandcastle
Reviewed By: SS-JIA
Differential Revision: D27225941
fbshipit-source-id: 8b5439b6bf5e24ea755cb1941d92fae26a8d5a06
