add export_model.py (#3842)

Summary:
In this series of PRs, I am going to enable the [Phi-3-mini](https://huggingface.co/microsoft/Phi-3-mini-128k-instruct) model to run on executorch. This is the [issue](https://github.com/pytorch/executorch/issues/3550?fbclid=IwAR3jsO1ZAuLXLDSqiCAYLxhRsT-RWTsnRTnQAH1QthEm5AHEaq0goTyz5Iw#issuecomment-2122973779) for this work.

This diff adds the minimal script for exporting and quantizing the model to executorch. In later PRs, I will refactor it to reuse the `export_lib` used by lama2.

Pull Request resolved: https://github.com/pytorch/executorch/pull/3842

Reviewed By: larryliu0820

Differential Revision: D58167845

Pulled By: helunwencser

fbshipit-source-id: 66ccf3316984b41505676d5f479c2ea6302a2434