Add instructions to convert Hugging Face models to PyTorch (#3523)

Summary:
As titled. It's pretty common that users download the LLM models in safetensor format. Add instructions and example script to convert them to PyTorch format so that export_llama script can accept. It leverages the utils from TorchTune.

Thanks l3utterfly and kartikayk for the discussions and suggestions!

More context in https://github.com/pytorch/executorch/issues/3303

Pull Request resolved: https://github.com/pytorch/executorch/pull/3523

Reviewed By: mergennachin

Differential Revision: D57026658

Pulled By: iseeyuan

fbshipit-source-id: 11badf709920ff945cdfdd2b244c52c750943412