`RetrievalQAWithSourcesChain` not returning sources in `sources` field. 

### System Info

System Info (Docker Dev Container):
```
PRETTY_NAME="Debian GNU/Linux 10 (buster)"
NAME="Debian GNU/Linux"
VERSION_ID="10"
VERSION="10 (buster)"
VERSION_CODENAME=buster
ID=debian
HOME_URL="https://www.debian.org/"
SUPPORT_URL="https://www.debian.org/support"
BUG_REPORT_URL="https://bugs.debian.org/"
```

Python: 3.10

Pip:
```
absl-py                  1.4.0
aiohttp                  3.8.4
aiosignal                1.3.1
antlr4-python3-runtime   4.9.3
anyio                    3.6.2
argilla                  1.6.0
async-timeout            4.0.2
attrs                    23.1.0
backoff                  2.2.1
cachetools               5.3.0
certifi                  2022.12.7
cffi                     1.15.1
charset-normalizer       3.1.0
click                    8.1.3
cloudpickle              2.2.1
cmake                    3.26.3
coloredlogs              15.0.1
commonmark               0.9.1
contourpy                1.0.7
cryptography             40.0.2
cycler                   0.11.0
dataclasses-json         0.5.7
Deprecated               1.2.13
detectron2               0.4
effdet                   0.3.0
et-xmlfile               1.1.0
exceptiongroup           1.1.1
fastapi                  0.95.1
filelock                 3.11.0
flatbuffers              23.3.3
fonttools                4.39.3
frozenlist               1.3.3
future                   0.18.3
fvcore                   0.1.3.post20210317
google-auth              2.17.3
google-auth-oauthlib     1.0.0
gptcache                 0.1.11
greenlet                 2.0.2
grpcio                   1.53.0
h11                      0.14.0
httpcore                 0.16.3
httpx                    0.23.3
huggingface-hub          0.13.4
humanfriendly            10.0
idna                     3.4
iniconfig                2.0.0
iopath                   0.1.10
Jinja2                   3.1.2
joblib                   1.2.0
kiwisolver               1.4.4
langchain                0.0.141
layoutparser             0.3.4
lit                      16.0.1
lxml                     4.9.2
Markdown                 3.4.3
MarkupSafe               2.1.2
marshmallow              3.19.0
marshmallow-enum         1.5.1
matplotlib               3.7.1
monotonic                1.6
mpmath                   1.3.0
msg-parser               1.2.0
multidict                6.0.4
mypy-extensions          1.0.0
networkx                 3.1
nltk                     3.8.1
numpy                    1.23.5
nvidia-cublas-cu11       11.10.3.66
nvidia-cuda-cupti-cu11   11.7.101
nvidia-cuda-nvrtc-cu11   11.7.99
nvidia-cuda-runtime-cu11 11.7.99
nvidia-cudnn-cu11        8.5.0.96
nvidia-cufft-cu11        10.9.0.58
nvidia-curand-cu11       10.2.10.91
nvidia-cusolver-cu11     11.4.0.1
nvidia-cusparse-cu11     11.7.4.91
nvidia-nccl-cu11         2.14.3
nvidia-nvtx-cu11         11.7.91
oauthlib                 3.2.2
olefile                  0.46
omegaconf                2.3.0
onnxruntime              1.14.1
openai                   0.27.4
openapi-schema-pydantic  1.2.4
opencv-python            4.6.0.66
openpyxl                 3.1.2
packaging                23.1
pandas                   1.5.3
pdf2image                1.16.3
pdfminer.six             20221105
pdfplumber               0.9.0
pgvector                 0.1.6
Pillow                   9.5.0
pip                      23.1
pluggy                   1.0.0
portalocker              2.7.0
protobuf                 4.22.3
psycopg2-binary          2.9.6
pyasn1                   0.4.8
pyasn1-modules           0.2.8
pycocotools              2.0.6
pycparser                2.21
pydantic                 1.10.7
pydot                    1.4.2
Pygments                 2.15.0
pypandoc                 1.11
pyparsing                3.0.9
pypdf                    3.9.0
pytesseract              0.3.10
pytest                   7.3.1
python-dateutil          2.8.2
python-docx              0.8.11
python-dotenv            1.0.0
python-magic             0.4.27
python-multipart         0.0.6
python-poppler           0.4.0
python-pptx              0.6.21
pytz                     2023.3
PyYAML                   6.0
regex                    2023.3.23
requests                 2.28.2
requests-oauthlib        1.3.1
rfc3986                  1.5.0
rich                     13.0.1
rsa                      4.9
scipy                    1.10.1
setuptools               65.5.1
six                      1.16.0
sniffio                  1.3.0
SQLAlchemy               1.4.47
starlette                0.26.1
sympy                    1.11.1
tabulate                 0.9.0
tenacity                 8.2.2
tensorboard              2.12.2
tensorboard-data-server  0.7.0
tensorboard-plugin-wit   1.8.1
termcolor                2.2.0
tiktoken                 0.3.3
timm                     0.6.13
tokenizers               0.13.3
tomli                    2.0.1
torch                    2.0.0
torchaudio               2.0.1
torchvision              0.15.1
tqdm                     4.65.0
transformers             4.28.1
triton                   2.0.0
typing_extensions        4.5.0
typing-inspect           0.8.0
unstructured             0.5.12
unstructured-inference   0.3.2
urllib3                  1.26.15
uvicorn                  0.21.1
Wand                     0.6.11
Werkzeug                 2.2.3
wheel                    0.40.0
wrapt                    1.14.1
XlsxWriter               3.1.0
yacs                     0.1.8
yarl                     1.8.2
```

### Who can help?

@hwchase17 

### Information

- [ ] The official example notebooks/scripts
- [X] My own modified scripts

### Related Components

- [X] LLMs/Chat Models
- [ ] Embedding Models
- [ ] Prompts / Prompt Templates / Prompt Selectors
- [ ] Output Parsers
- [ ] Document Loaders
- [ ] Vector Stores / Retrievers
- [ ] Memory
- [ ] Agents / Agent Executors
- [ ] Tools / Toolkits
- [X] Chains
- [ ] Callbacks/Tracing
- [ ] Async

### Reproduction

Write the code below:
```
      chain = RetrievalQAWithSourcesChain.from_chain_type(
            llm=ChatOpenAI(openai_api_key=api_key),
            chain_type="map_reduce",
            retriever=retriever,
        )

        llm_call = "random llm call"

        result = chain({
            "question": llm_call,
        },
            return_only_outputs=True
        )
```

### Expected behavior

I'm expecting that I'll be having a `result["answer"]` and non empty `result["sources"]` but here's what I get instead:
![image](https://github.com/hwchase17/langchain/assets/69240261/a49a54eb-315c-45d5-a4e7-a647c3d818e3)

As you can see, `sources` is empty but it's included in `result["answer"]` as a string.