Move builder.py into extension (#4188)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/4188

This PR does the following things:
- rename `LlamaEdgeManager` to `LLMEdgeManager`
- clean up some unused parameters: `weight_type`, `use_sdpa_with_kv_cache`
- move `model.params.max_seq_len` out of `builder.py`
- move `builder.py` into `extension/llm/export`
ghstack-source-id: 233306023
exported-using-ghexport

Reviewed By: larryliu0820

Differential Revision: D59493975

fbshipit-source-id: 47cbdbddff361efcefe4555b046e63cebdf367ec