Mark extension/llm examples/llama2 and examples/llava C++ APIs experimental (#6125)

Summary:
experimental

As titled.

Pull Request resolved: https://github.com/pytorch/executorch/pull/6125

Reviewed By: lucylq

Differential Revision: D64198599

Pulled By: larryliu0820

fbshipit-source-id: c2540fbebf255347467302a61112784e5b5e5b61