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",
+        "<_>"
+    ]
+}