Enable QNNPACK for iOS (#24030)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/24030
The cmake arg - `USE_QNNPACK` was disabled for iOS build due to its lack of support for building multiple archs(armv7;armv7s;arm64) simultaneously.To enable it, we need to specify the value of IOS_ARCH explicitly in the build command:
```
./scripts/build_ios.sh \
-DIOS_ARCH=arm64 \
-DBUILD_CAFFE2_MOBILE=OFF \
```
However,the iOS.cmake will overwirte this value according to the value of `IOS_PLATFORM`. This PR is a fix to this problem.
Test Plan:
- `USE_QNNPACK` should be turned on by cmake.
- `libqnnpack.a` can be generated successfully.
- `libortch.a` can be compiled and run successfully on iOS devices.
<img src="https://github.com/xta0/AICamera-ObjC/blob/master/aicamera.gif?raw=true" width="400">
Differential Revision: D16771014
Pulled By: xta0
fbshipit-source-id: 4cdfd502cb2bcd29611e4c22e2efdcdfe9c920d3
