Make export_llm as a separate binary

[kimishpatel]

🚀 The feature, motivation and pitch

Today users invoke python examples.models.llama.export_llama... to export model. It would be nice to have a set of binary utils installed as part of pip install executorch that can be used for for model export and lowering.

Alternatives

Continue to use python examples.models.llama.export_llama...

Additional context

This may have some overlap with export wizard @byjlw, however I am asking this one to focus more on generative ai usecases.

RFC (Optional)

No response

cc @mergennachin @iseeyuan @lucylq @helunwencser @tarun292 @jackzhxng