doc: emphasize and notify the usage of chat_template (#3589)

Co-authored-by: Chayenne <zhaochen20@outlook.com>

python/sglang/srt/openai_api/adapter.py

@@ -20,12 +20,14 @@ import os
 import time
 import uuid
 from http import HTTPStatus
-from typing import Dict, List, Optional
+from typing import Dict, List
 
 from fastapi import HTTPException, Request, UploadFile
 from fastapi.responses import ORJSONResponse, StreamingResponse
 from pydantic import ValidationError
 
+from sglang.lang.chat_template import get_chat_template_by_model_path
+
 try:
     from outlines.fsm.json_schema import convert_json_schema_to_str
 except ImportError:
@@ -92,7 +94,6 @@ file_id_response: Dict[str, FileResponse] = {}
 # map file id to file path in SGLang backend
 file_id_storage: Dict[str, str] = {}
 
-
 # backend storage directory
 storage_dir = None
 
@@ -116,12 +117,13 @@ def create_streaming_error_response(
     return json_str
 
 
-def load_chat_template_for_openai_api(tokenizer_manager, chat_template_arg):
+def load_chat_template_for_openai_api(tokenizer_manager, chat_template_arg, model_path):
     global chat_template_name
 
     logger.info(
         f"Use chat template for the OpenAI-compatible API server: {chat_template_arg}"
     )
+
     if not chat_template_exists(chat_template_arg):
         if not os.path.exists(chat_template_arg):
             raise RuntimeError(
@@ -163,6 +165,18 @@ def load_chat_template_for_openai_api(tokenizer_manager, chat_template_arg):
     else:
         chat_template_name = chat_template_arg
 
+    # check chat-template
+    chat_template = get_chat_template_by_model_path(model_path)
+    if chat_template is not None:
+        official_chat_template = chat_template.name
+        used_chat_template = chat_template_name
+        if official_chat_template != used_chat_template:
+            logger.warning(
+                f"Using a chat_template: '{used_chat_template}', "
+                f"which is different from official chat template: '{official_chat_template}', "
+                f"This discrepancy may lead to performance degradation."
+            )
+
 
 async def v1_files_create(file: UploadFile, purpose: str, file_storage_pth: str = None):
     try: