fix deprecated top_k for GPTAPI (#185)

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

lagent/llms/openai.py

@@ -67,6 +67,7 @@ def __init__(self,
             query_per_second=query_per_second,
             retry=retry,
             **gen_params)
+        self.gen_params.pop('top_k')
         self.logger = getLogger(__name__)
 
         if isinstance(key, str):