Print the number of tokens generated (#6773)

Pull Request resolved: https://github.com/pytorch/executorch/pull/6771

This is useful for verifying the correctness of AttentionSink.
ghstack-source-id: 252993225
@exported-using-ghexport

Differential Revision: [D65784095](https://our.internmc.facebook.com/intern/diff/D65784095/)

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