Fix Cuda out of memory issue for eager runner (#6935)

Pull Request resolved: https://github.com/pytorch/executorch/pull/6866

This PR updates the eager runner to disable grad and save memory usage.

It also update the prompt format to not include bos.
ghstack-source-id: 254139542

Differential Revision: [D65962743](https://our.internmc.facebook.com/intern/diff/D65962743/)

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