Add additional_special_tokens

tokenizer_config.json

@@ -5,5 +5,10 @@
     "sep_token": "</s>",
     "cls_token": "<s>",
     "pad_token": "<pad>",
-    "mask_token": "<mask>"
+    "mask_token": "<mask>",
+    "additional_special_tokens": [
+         "<s>NOTUSED",
+         "</s>NOTUSED",
+         "<_>"
+    ]
 }