Refactor export_llama_lib (#2030)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/2030

Separate out "receipe" code and actual "cooking" code.

Introduces a new `LlamaBuilder` class that handles internal logic of exporting. Takes source transforms, quantizers, and partitioners.

Reviewed By: mergennachin

Differential Revision: D54027081

fbshipit-source-id: 8da52b7a538331389eb148bff1b143373fcefb18