add benchmark code to iOS TestApp (#28405)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/28405
### Summary
As discussed with AshkanAliabadi and ljk53, the iOS TestApp will share the same benchmark code with Android's speed_benchmark_torch.cpp. This PR is the first part which contains the Objective-C++ code.
The second PR will include the scripts to setup and run the benchmark project. The third PR will include scripts that can automate the whole "build - test - install" process.
There are many ways to run the benchmark project. The easiest way is to use cocoapods. Simply run `pod install`. However, that will pull the 1.3 binary which is not what we want, but we can still use this approach to test the benchmark code. The second PR will contain scripts to run custom builds that we can tweak.
### Test Plan
- Don't break any existing CI jobs (except for those flaky ones)
Test Plan: Imported from OSS
Differential Revision: D18064187
Pulled By: xta0
fbshipit-source-id: 4cfbb83c045803d8b24bf6d2c110a55871d22962
