[ET-VK] Add util to extract VkGraph from Program delegate data

[ET-VK] Add util to extract VkGraph from Program

Pull Request resolved: https://github.com/pytorch/executorch/pull/8199

Missing piece to facilitate VkGraph debugging.

Differential Revision: [D69151587](https://our.internmc.facebook.com/intern/diff/D69151587/)
ghstack-source-id: 264726843

Co-authored-by: jorgep31415 <jorgepineda140@gmail.com>