add phi-3-mini runner (#3951)

Summary:
This PR adds a basic runner for running the phi-3-mini model. It uses sentencepiece to create the tokenizer.

Commands for running the model:
```
# setup executorch per instructions in https://pytorch.org/executorch/stable/getting-started-setup.html
# install latest transformers
pip uninstall -y transformers && pip install git+https://github.com/huggingface/transformers
# export the model, will take a few minutes
cd examples/models/phi-3-mini
python export_model.py
# download the tokenizer.model
wget -O tokenizer.model https://huggingface.co/microsoft/Phi-3-mini-128k-instruct/resolve/main/tokenizer.model?download=true
# build the runner
mkdir cmake-out
cd cmake-out
cmake ..
cd ..
cmake --build cmake-out -j10
./cmake-out/phi_3_mini_runner
```

Pull Request resolved: https://github.com/pytorch/executorch/pull/3951

Reviewed By: larryliu0820

Differential Revision: D58477481

Pulled By: helunwencser

fbshipit-source-id: c5a7e6781338d4347a1b9d06b22e23613633df6b