enable loading Mbart50Tokenizer with AutoTokenizer (#10690)

* enable auto tokenizer for mbart50 tokenizers

* fix imports

src/transformers/models/auto/tokenization_auto.py

@@ -116,6 +116,7 @@ if is_sentencepiece_available():
     from ..m2m_100 import M2M100Tokenizer
     from ..marian.tokenization_marian import MarianTokenizer
     from ..mbart.tokenization_mbart import MBartTokenizer
+    from ..mbart.tokenization_mbart50 import MBart50Tokenizer
     from ..mt5 import MT5Tokenizer
     from ..pegasus.tokenization_pegasus import PegasusTokenizer
     from ..reformer.tokenization_reformer import ReformerTokenizer
@@ -132,6 +133,7 @@ else:
     DebertaV2Tokenizer = None
     MarianTokenizer = None
     MBartTokenizer = None
+    MBart50Tokenizer = None
     MT5Tokenizer = None
     PegasusTokenizer = None
     ReformerTokenizer = None
@@ -159,6 +161,7 @@ if is_tokenizers_available():
     from ..led.tokenization_led_fast import LEDTokenizerFast
     from ..longformer.tokenization_longformer_fast import LongformerTokenizerFast
     from ..lxmert.tokenization_lxmert_fast import LxmertTokenizerFast
+    from ..mbart.tokenization_mbart50_fast import MBart50TokenizerFast
     from ..mbart.tokenization_mbart_fast import MBartTokenizerFast
     from ..mobilebert.tokenization_mobilebert_fast import MobileBertTokenizerFast
     from ..mpnet.tokenization_mpnet_fast import MPNetTokenizerFast
@@ -190,6 +193,7 @@ else:
     LongformerTokenizerFast = None
     LxmertTokenizerFast = None
     MBartTokenizerFast = None
+    MBart50TokenizerFast = None
     MobileBertTokenizerFast = None
     MPNetTokenizerFast = None
     MT5TokenizerFast = None
@@ -268,6 +272,8 @@ NO_CONFIG_TOKENIZER = [
     PhobertTokenizer,
     BarthezTokenizer,
     BarthezTokenizerFast,
+    MBart50Tokenizer,
+    MBart50TokenizerFast,
 ]