update llama runner to decode single token (#6768)

Pull Request resolved: https://github.com/pytorch/executorch/pull/6703

Right now, we don't print the generated response in the eager runner until all tokens are generated. This is not good experience as we need to wait until all tokens are generated to see the response.

This PR updates it to decode each new token immediately after it is generated.
ghstack-source-id: 252924039

Differential Revision: [D65578306](https://our.internmc.facebook.com/intern/diff/D65578306/)

Co-authored-by: Lunwen He <lwhecser@gmail.com>