Fix the iOS build (#23293)
Summary:
The legacy iOS build script (`build_ios.sh`) is still working, but the output is in caffe2, not Pytorch. To enable the Pytorch iOS build, we can set the value of `BUILD_CAFFE2_MOBILE` to `NO`, and turn on another cmake arg - `INTERN_BUILD_MOBILE` ljk53 has created for Android.
There is a trivial issue in `used_kernel.cpp` that will cause the compiling error when running `build_ios.sh`, as it uses a `system`API that has been deprecated since iOS 11. The fix below is to bypass this file since it's not needed by mobile.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/23293
Test Plan:
The `build_ios.sh` completed successfully, and all the generated static libraries can be compiled and linked successfully on iOS devices.
### Build script
```shell
./scripts/build_ios.sh \
-DBUILD_CAFFE2_MOBILE=OFF \
-DCMAKE_PREFIX_PATH=$(python -c 'from distutils.sysconfig import get_python_lib; print(get_python_lib())') \
-DPYTHON_EXECUTABLE=$(python -c 'import sys; print(sys.executable)')
```
Differential Revision: D16456100
Pulled By: xta0
fbshipit-source-id: 38c73e1e3a0c219a38ddc28b31acc181690f34e8
