Multilingual BART - (#3602)

- support mbart-en-ro weights
- add MBartTokenizer

src/transformers/__init__.py

@@ -122,7 +122,7 @@ from .pipelines import (
 )
 from .tokenization_albert import AlbertTokenizer
 from .tokenization_auto import TOKENIZER_MAPPING, AutoTokenizer
-from .tokenization_bart import BartTokenizer
+from .tokenization_bart import BartTokenizer, MBartTokenizer
 from .tokenization_bert import BasicTokenizer, BertTokenizer, BertTokenizerFast, WordpieceTokenizer
 from .tokenization_bert_japanese import BertJapaneseTokenizer, CharacterTokenizer, MecabTokenizer
 from .tokenization_camembert import CamembertTokenizer