fix: token counting for chat openai. (#2543)

I noticed that the value of get_num_tokens_from_messages in `ChatOpenAI`
is always one less than the response from OpenAI's API. Upon checking
the official documentation, I found that it had been updated, so I made
the necessary corrections.
Then now I got the same value from OpenAI's API.


d972e7482e (diff-2d4485035b3a3469802dbad11d7b4f834df0ea0e2790f418976b303bc82c1874L474)

langchain/chat_models/openai.py

@@ -400,5 +400,5 @@ class ChatOpenAI(BaseChatModel):
                 if key == "name":
                     num_tokens += tokens_per_name
         # every reply is primed with <im_start>assistant
-        num_tokens += 2
+        num_tokens += 3
         return num_tokens