GitLab

* Copy BART to MBart and rename some stuff

* Add copy statements pointing to FlaxBart

* Update/add some common files

* Update shift_tokens_rigth + fix imports

* Fix shift_tokens_right method according to MBart implementation

* Update shift_tokens_right in tests accordingly

* Fix the import issue and update docs file
* make style quality

* Do some minor changes according to patil-suraj suggestions

* Change the order of normalization layer and attention

* Add some copu statementes

* Update generate method and add integration test for mBart

* Make a few updates after a review

Besides, add `lang_code_to_id` to MBartTokenizeFast

* fix-copies; make style quality

* Apply suggestions from code review

* Apply suggestions from code review

* Apply suggestions from code review

* fix output type, style

* add copied from

* resolve conflicts
Co-authored-by: Suraj Patil <surajp815@gmail.com>