Allow RAG to output decoder cross-attentions (#9789)
* get cross attns
* add cross-attns doc strings
* fix typo
* line length
* Apply suggestions from code review
Co-authored-by: Quentin Lhoest <42851186+lhoestq@users.noreply.github.com>
Co-authored-by: Quentin Lhoest <42851186+lhoestq@users.noreply.github.com>
