Fix conv_template: use full object instead of string name (fixes web-llm compat)

mlc-chat-config.json

@@ -11,7 +11,30 @@
   "rope_theta": 1000000.0,
   "vocab_size": 151936,
   "tokenizer_files": ["tokenizer.json", "tokenizer_config.json"],
-  "conv_template": "qwen2",
+  "conv_template": {
+    "name": "qwen2",
+    "system_template": "<|im_start|>system\n{system_message}<|im_end|>\n",
+    "system_message": "You are a helpful assistant.",
+    "system_prefix_token_ids": null,
+    "add_role_after_system_message": true,
+    "roles": {
+      "user": "<|im_start|>user",
+      "assistant": "<|im_start|>assistant"
+    },
+    "role_templates": {
+      "user": "{user_message}",
+      "assistant": "{assistant_message}",
+      "tool": "{tool_message}"
+    },
+    "messages": [],
+    "seps": ["<|im_end|>\n"],
+    "role_content_sep": "\n",
+    "role_empty_sep": "\n",
+    "stop_str": ["<|endoftext|>", "<|im_end|>"],
+    "stop_token_ids": [151643, 151645],
+    "function_string": "",
+    "use_function_calling": false
+  },
   "context_window_size": 4096,
   "prefill_chunk_size": 1024,
   "sliding_window_size": -1,