Add mBART-50 (#10154)

* add tokenizer for mBART-50

* update tokenizers

* make src_lang and tgt_lang optional

* update tokenizer test

* add setter

* update docs

* update conversion script

* update docs

* update conversion script

* update tokenizer

* update test

* update docs

* doc

* address Sylvain's suggestions

* fix test

* fix formatting

* nits

src/transformers/utils/dummy_tokenizers_objects.py

@@ -164,6 +164,15 @@ class LxmertTokenizerFast:
         requires_tokenizers(self)
 
 
+class MBart50TokenizerFast:
+    def __init__(self, *args, **kwargs):
+        requires_tokenizers(self)
+
+    @classmethod
+    def from_pretrained(self, *args, **kwargs):
+        requires_tokenizers(self)
+
+
 class MBartTokenizerFast:
     def __init__(self, *args, **kwargs):
         requires_tokenizers(self)