[iOS GPU] Fix Shader compilation errors for Metal 1.2 (iOS 12) (#56670)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/56670
`int64_t` is only available for Metal 2.2 and above. `size_t` works fine in those situations. https://developer.apple.com/metal/Metal-Shading-Language-Specification.pdf
ghstack-source-id: 127169610
Test Plan:
- AIBench
```
buck run mode/mac aibench:run_bench_macos -- -b aibench/specifications/models/pytorch/metal/metal_unet_1001_detection.json --platform ios --framework pytorch --remote --devices D201 (https://github.com/pytorch/pytorch/commit/85b1c45a456591bafd51ca79b8a03cca59f90fde)AP-12.0.1
```
Reviewed By: linbinyu
Differential Revision: D27933297
fbshipit-source-id: 474b1eb191c68101367c9623c855645684434bd7
