Add improved sources splitting in BaseQAWithSourcesChain
Labels: name: needs test, name: auto:improvement

## Type:
Improvement

---

## Description:
Running QAWithSourcesChain sometimes raises ValueError as mentioned in issue #7184:
```
ValueError: too many values to unpack (expected 2)
Traceback:

    response = qa({"question": pregunta}, return_only_outputs=True)
File "C:\Anaconda3\envs\iagen_3_10\lib\site-packages\langchain\chains\base.py", line 166, in __call__
    raise e
File "C:\Anaconda3\envs\iagen_3_10\lib\site-packages\langchain\chains\base.py", line 160, in __call__
    self._call(inputs, run_manager=run_manager)
File "C:\Anaconda3\envs\iagen_3_10\lib\site-packages\langchain\chains\qa_with_sources\base.py", line 132, in _call
    answer, sources = re.split(r"SOURCES:\s", answer)
```
This is due to LLM model generating subsequent question, answer and sources, that is complement in a similar form as below:
```
<final_answer>
SOURCES: <sources>
QUESTION: <new_or_repeated_question>
FINAL ANSWER: <new_or_repeated_final_answer>
SOURCES: <new_or_repeated_sources>
```
It leads the following line
```
 re.split(r"SOURCES:\s", answer)
```
to return more than 2 elements and result in ValueError. The simple fix is to split also with "QUESTION:\s" and take the first two elements:
```
answer, sources = re.split(r"SOURCES:\s|QUESTION:\s", answer)[:2]
```

Sometimes LLM might also generate some other texts, like alternative answers in a form:
```
<final_answer_1>
SOURCES: <sources>

<final_answer_2>
SOURCES: <sources>

<final_answer_3>
SOURCES: <sources>
```
In such cases it is the best to split previously obtained sources with new line:
```
sources = re.split(r"\n", sources.lstrip())[0]
```



---

## Issue:
Resolves #7184

---

## Maintainer:
@baskaryan


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