refactor runner to fix bugs (#2752)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/2752

Lots of bugs before with kv cache implementation of runner. Trying this out

The bugs were mostly various ways that the prompt sequence was getting trashed in kv cache mode

Reviewed By: iseeyuan

Differential Revision: D55496044

fbshipit-source-id: c98cbb26ce022981eb7eddbadd57037bad94739a