Fix an unusual issue that occurs when using OpenAIChat for llm_math (#1410)

Fix an issue that occurs when using OpenAIChat for llm_math, refer to
the code style of the "Final Answer:" in Mrkl。 the reason is I found a
issue when I try OpenAIChat for llm_math, when I try the question in
Chinese, the model generate the format like "\n\nQuestion: What is the
square of 29?\nAnswer: 841", it translate the question first , then
answer. below is my snapshot:
<img width="945" alt="snapshot"
src="https://user-images.githubusercontent.com/82029664/222642193-10ecca77-db7b-4759-bc46-32a8f8ddc48f.png">

langchain/chains/llm_math/base.py

@@ -62,6 +62,8 @@ class LLMMathChain(Chain, BaseModel):
             answer = "Answer: " + output
         elif t.startswith("Answer:"):
             answer = t
+        elif "Answer:" in t:
+            answer = "Answer: " + t.split("Answer:")[-1]
         else:
             raise ValueError(f"unknown format from LLM: {t}")
         return {self.output_key: answer}