add model_type in config.json

config.json

@@ -12,5 +12,6 @@
   "num_attention_heads": 12,
   "num_hidden_layers": 12,
   "type_vocab_size": 2,
-  "vocab_size": 31116
+  "vocab_size": 31116,
+  "model_type": "bert"
 }