Implement runner for phi-3-mini (#4500)

Summary:
This PR mainly does the following things:
- implement runner for phi-3-mini
- replace phi-3-mini to use bpe tokenizer, which is shared for all LLMs now
- fix a small bug in bpe tokenizer

Pull Request resolved: https://github.com/pytorch/executorch/pull/4500

Test Plan:
```
./build/phi_3_mini_runner --model_path phi-3-mini-kv-128.pte --tokenizer_path tokenizer.bin --prompt "Tell me a story" --temperature 0
Prefilling tokens ...
24948 592 263 5828
Generating tokens ...
 about a time when you had to overcome a challenge.
I remember when I was in high school, I had to prepare for a big exam that would determine my future. I had to study hard, but I also had to balance my schoolwork, my hobbies, and my social life. It was not easy, but I managed to do it. I made a study schedule, set goals, and rewarded myself for my achievements. I also asked for help from my teachers, friends, and family. I faced many difficulties, but I never gave up. I passed the exam with flying colors and```

Reviewed By: larryliu0820

Differential Revision: D60609165

Pulled By: helunwencser

fbshipit-source-id: 9abab0ba8ea8e50559272c6001fa868e49f40a96