Cocoapods for iOS OSS release (#25847)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/25847
### Summary
The Podspec file for iOS OSS release. This podspec contains the C++ header files and a static library that supports three architectures.
Please ignore the link for `s.source` for now, as I'm still working on the CI nightly build. This is a temporary link for testing purpose.
### Note
Previously I have a cocoapods release proposal - https://github.com/pytorch/pytorch/pull/25543 which contains two podspec files. However, for the time being, we haven't decided whether we want to release the Objective-C API wrapper or not. Please review and refer to this one if you have questions.
Test Plan: Imported from OSS
Differential Revision: D17262459
Pulled By: xta0
fbshipit-source-id: 4cc60787a41beab14cf9b1c0e9ab62b8b14603c5
