Transform model to be able to use Attention Sink

Pull Request resolved: https://github.com/pytorch/executorch/pull/6700

This PR adds necessary functions for transforming the model to be able to use Attention Sink.
ghstack-source-id: 256108077
@exported-using-ghexport

Differential Revision: [D65571289](https://our.internmc.facebook.com/intern/diff/D65571289/)

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