move code under executorch/example (#3176)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/3176
This diff moves llm manual code from outside github (Dave's and Georgey's) to executorch codebase for better pointing to.
After this diff. //executorch/examples/llm_maunal will become the only source of truth of our llm manual code.

Reviewed By: byjlw, dbort

Differential Revision: D56365058

fbshipit-source-id: 97280fc0ca955caabb6056cddbb72102ed711f2c