Allow no token limit for ChatGPT API (#1481)

The endpoint default is inf if we don't specify max_tokens, so unlike
regular completion API, we don't need to calculate this based on the
prompt.

langchain/llms/openai.py

@@ -619,6 +619,9 @@ class OpenAIChat(BaseLLM, BaseModel):
             if "stop" in params:
                 raise ValueError("`stop` found in both the input and default params.")
             params["stop"] = stop
+        if params.get("max_tokens") == -1:
+            # for ChatGPT api, omitting max_tokens is equivalent to having no limit
+            del params["max_tokens"]
         return messages, params
 
     def _generate(