change chat default (#1782)

Resolves #1532, resolves
#1652.

langchain/chat_models/openai.py

@@ -128,7 +128,7 @@ class ChatOpenAI(BaseChatModel, BaseModel):
     """Whether to stream the results or not."""
     n: int = 1
     """Number of chat completions to generate for each prompt."""
-    max_tokens: int = 256
+    max_tokens: Optional[int] = None
     """Maximum number of tokens to generate."""
 
     class Config: