Fix returns of OpenAI interface (#108)

fix `BaseAPIModel` chat returns

Co-authored-by: wangzy <[email protected]>

lagent/llms/openai.py

@@ -99,7 +99,7 @@ def chat(
         self,
         inputs: Union[List[dict], List[List[dict]]],
         **gen_params,
-    ) -> List[str]:
+    ) -> Union[str, List[str]]:
         """Generate responses given the contexts.
 
         Args:
@@ -108,7 +108,7 @@ def chat(
             gen_params: additional generation configuration
 
         Returns:
-            List[str]: A list of generated strings.
+            Union[str, List[str]]: generated string(s)
         """
         assert isinstance(inputs, list)
         if isinstance(inputs[0], dict):
@@ -120,7 +120,8 @@ def chat(
                 for messages in inputs
             ]
         wait(tasks)
-        return [task.result() for task in tasks]
+        ret = [task.result() for task in tasks]
+        return ret[0] if isinstance(inputs[0], dict) else ret
 
     def _chat(self, messages: List[dict], **gen_params) -> str:
         """Generate completion from a list of templates.