Allow send list of str for the Prompt on openai demo endpoint /v1/completions (#323)

* allow str or List[str] for prompt

* Update vllm/entrypoints/openai/api_server.py

Co-authored-by: Zhuohan Li <zhuohan123@gmail.com>

---------

Co-authored-by: Zhuohan Li <zhuohan123@gmail.com>

vllm/entrypoints/openai/api_server.py

@@ -357,7 +357,11 @@ async def create_completion(raw_request: Request):
 
     model_name = request.model
     request_id = f"cmpl-{random_uuid()}"
-    prompt = request.prompt
+    if isinstance(request.prompt, list):
+        assert len(request.prompt) == 1
+        prompt = request.prompt[0]
+    else:
+        prompt = request.prompt
     created_time = int(time.time())
     try:
         sampling_params = SamplingParams(