[Metal] Do not use read/write textures in concat shaders (#61074)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/61074
`read_write` textures are not available on some devices, such as iPhone 7. This prevents the concat op from functioning on those devices.
This diff rewrites the concat shaders such that they do not depend on `read_write` textures.
Test Plan:
Test on device: run squeezenet and/or the operator tests
```
arc focus2 pp-ios
```
Test on Mac
```
buck test pp-macos
```
Test specifically on iPhone7, either device or simulator.
Reviewed By: xta0
Differential Revision: D29501656
fbshipit-source-id: de4a059953ab4b0abf38b6ecb3f665323dcdbea1
