Add testing script for iOS x86 build (#26632)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/26632
### Summary
This script builds the TestApp (located in ios folder) to generate an iOS x86 executable via the `xcodebuild` toolchain on macOS. The goal is to provide a quick way to test the generated static libraries to see if there are any linking errors. The script can also be used by the iOS CI jobs. To run the script, simply see description below:
```shell
$ruby scripts/xcode_ios_x86_build.rb --help
-i, --install path to the cmake install folder
-x, --xcodeproj path to the XCode project file
```
### Note
The script mainly deals with the iOS simulator build. For the arm64 build, I haven't found a way to disable the Code Sign using the `xcodebuiild` tool chain (XCode 10). If anyone knows how to do that, please feel free to leave a comment below.
### Test Plan
- The script can build the TestApp and link the generated static libraries successfully
- Don't break any CI job
Test Plan: Imported from OSS
Differential Revision: D17530990
Pulled By: xta0
fbshipit-source-id: f50bef7127ff8c11e884c99889cecff82617212b
