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>

src/transformers/utils/dummy_tokenizers_objects.py

@@ -20,6 +20,15 @@ class BartTokenizerFast:
         requires_tokenizers(self)
 
 
+class BarthezTokenizerFast:
+    def __init__(self, *args, **kwargs):
+        requires_tokenizers(self)
+
+    @classmethod
+    def from_pretrained(self, *args, **kwargs):
+        requires_tokenizers(self)
+
+
 class BertTokenizerFast:
     def __init__(self, *args, **kwargs):
         requires_tokenizers(self)