Setting vocab_size to solve the issue of mlc-chat module not initalizing

Solves the issue of vocab_size requirement when initializing ChatModule()

TVMError: Check failed: (partial_update) is false: Key "vocab_size" not found.

params/mlc-chat-config.json

@@ -13,5 +13,6 @@
         "tokenizer.json"
     ],
     "model_category": "llama",
-    "model_name": "Llama-2-13b-chat-omniquant"
+    "model_name": "Llama-2-13b-chat-omniquant",
+    "vocab_size": 32000
 }