Move llama related stuff out of builder.py (#4105)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/4105

In the existing `builder.py`, `load_llama_model` and `_metadata` are llama specific code. To make `builder.py` sharable by other models, this diff moves these two functions into `export_llama_lib.py` such that `builder.py` is independent of llama.

Reviewed By: larryliu0820

Differential Revision: D59245601

fbshipit-source-id: 048c18688b64867a92c3e41ba04e892ccd9b1d72