speed up enforce_stop_tokens helper function (#10984)

**Description:**

As long as `enforce_stop_tokens` returns a first occurrence, we can
speed up the execution by setting the optional `maxsplit` parameter to
1.

Tag maintainer:
@agola11
@hwchase17

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Co-authored-by: Bagatur <baskaryan@gmail.com>

libs/langchain/langchain/llms/utils.py

@@ -5,4 +5,4 @@ from typing import List
 
 def enforce_stop_tokens(text: str, stop: List[str]) -> str:
     """Cut off the text as soon as any stop words occur."""
-    return re.split("|".join(stop), text)[0]
+    return re.split("|".join(stop), text, maxsplit=1)[0]