Add barthez model (#8393)
* Add init barthez
* Add barthez model, tokenizer and docs
BARThez is a pre-trained french seq2seq model that uses BART objective.
* Apply suggestions from code review docs typos
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>
* Add license
* Change URLs scheme
* Remove barthez model keep tokenizer
* Fix style
* Fix quality
* Update tokenizer
* Add fast tokenizer
* Add fast tokenizer test
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>
