Add Fill-in-Middle specific tokens

tokenizer_config.json

@@ -30,5 +30,8 @@
     "normalized": true,
     "rstrip": false,
     "single_word": false
-  }
+  },
+  "fim_prefix": "▁<PRE>",
+  "fim_middle": "▁<MID>",
+  "fim_suffix": "▁<SUF>"
 }