Specify tokenizer in config.

config.json

@@ -25,6 +25,7 @@
   "num_hidden_layers": 12,
   "num_memory_blocks": 0,
   "pad_token_id": 0,
+  "tokenizer_class": "BertTokenizerFast",
   "type_vocab_size": 2,
   "vocab_size": 21128
 }