Move iOS.cmake to the cmake folder (#24029)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/24029
The cmake toolchain file for building iOS is currently in `/third-pary/ios-cmake`. Since the upstream is not active anymore, It's better to maintain this file ourselves moving forward.This PR is also the prerequisite for enabling QNNPACK for iOS.
Test Plan:
- The `libtorch.a` can be generated successfully
- The `libtorch.a` can be compiled and run on iOS devices
<img src="https://github.com/xta0/AICamera-ObjC/blob/master/aicamera.gif?raw=true" width="400">
Differential Revision: D16770980
Pulled By: xta0
fbshipit-source-id: 1ed7b12b3699bac52b74183fa7583180bb17567e
