[from_pretrained] Allow tokenizer_type ≠ model_type (#6995)

src/transformers/configuration_utils.py

@@ -190,6 +190,7 @@ class PretrainedConfig(object):
             self.num_labels = kwargs.pop("num_labels", 2)
 
         # Tokenizer arguments TODO: eventually tokenizer and models should share the same config
+        self.tokenizer_class = kwargs.pop("tokenizer_class", None)
         self.prefix = kwargs.pop("prefix", None)
         self.bos_token_id = kwargs.pop("bos_token_id", None)
         self.pad_token_id = kwargs.pop("pad_token_id", None)