Increase `request_timeout` on ChatOpenAI (#3910)

With longer context and completions, gpt-3.5-turbo and, especially,
gpt-4, will more times than not take > 60seconds to respond.

Based on some other discussions, it seems like this is an increasingly
common problem, especially with summarization tasks.
- https://github.com/hwchase17/langchain/issues/3512
- https://github.com/hwchase17/langchain/issues/3005

OpenAI's max 600s timeout seems excessive, so I settled on 120, but I do
run into generations that take >240 seconds when using large prompts and
completions with GPT-4, so maybe 240 would be a better compromise?

langchain/chat_models/openai.py

@@ -120,7 +120,7 @@ class ChatOpenAI(BaseChatModel):
     """Holds any model parameters valid for `create` call not explicitly specified."""
     openai_api_key: Optional[str] = None
     openai_organization: Optional[str] = None
-    request_timeout: int = 60
+    request_timeout: int = 120
     """Timeout in seconds for the OpenAPI request."""
     max_retries: int = 6
     """Maximum number of retries to make when generating."""