Refactor cross attention and allow mechanism to tweak cross attention function (huggingface#1639)

* first proposal

* rename

* up

* Apply suggestions from code review

* better

* up

* finish

* up

* rename

* correct versatile

* up

* up

* up

* up

* fix

* Apply suggestions from code review

* make style

* Apply suggestions from code review

Co-authored-by: Pedro Cuenca <[email protected]>

* add error message

Co-authored-by: Pedro Cuenca <[email protected]>

Author: patrickvonplaten