ValidationError: 1 validation error for MapRerankDocumentsChain
Labels: name: bug

### System Info

* Langchain-0.0.215
* Python3.8.6

### Who can help?

@hwchase17 

### Information

- [ ] The official example notebooks/scripts
- [ ] My own modified scripts

### Related Components

- [ ] LLMs/Chat Models
- [ ] Embedding Models
- [X] Prompts / Prompt Templates / Prompt Selectors
- [ ] Output Parsers
- [ ] Document Loaders
- [X] Vector Stores / Retrievers
- [ ] Memory
- [ ] Agents / Agent Executors
- [ ] Tools / Toolkits
- [ ] Chains
- [ ] Callbacks/Tracing
- [ ] Async

### Reproduction



The chain type in `RetrievalQA.from_chain_type()`:
* `stuff`: work successfully
* `refine`: not work unless uses correctly naming or parameter. #6912
* `map_rerank`: not work
* `map_reduce`: not work

All the error of each type like: 

code:

```python
prompt_template = """
Use the following pieces of context to answer the question, if you don't know the answer, leave it blank don't try to make up an answer.

{context}

Question: {question}

Answer in JSON representations
"""

QA_PROMPT = PromptTemplate(
    template=prompt_template,
    input_variables=['context', 'question']
)
chain_type_kwargs = {
    'prompt': QA_PROMPT,
    'verbose': True
}

docs = PyMuPDFLoader('file.pdf').load()
splitter = RecursiveCharacterTextSplitter(
    chunk_size=chunk_size,
    chunk_overlap=chunk_overlap
)
docs = splitter.split_documents(document)

embeddings = OpenAIEmbeddings()
db = Chroma.from_documents(
    documents=docs,
    embedding= embeddings
)

qa_cahin = RetrievalQA.from_chain_type(
    llm=OpenAI(temperature=0.2),
    chain_type='map_rerank',
    retriever=db.as_retriever(),
    chain_type_kwargs=chain_type_kwargs
)
```

result:

```
ValidationError                           Traceback (most recent call last)
[c:\Users\JunXiang\AppData\Local\Programs\Python\Python38\lib\site-packages\langchain\chains\retrieval_qa\base.py](file:///C:/Users/JunXiang/AppData/Local/Programs/Python/Python38/lib/site-packages/langchain/chains/retrieval_qa/base.py) in from_chain_type(cls, llm, chain_type, chain_type_kwargs, **kwargs)
     89         """Load chain from chain type."""
     90         _chain_type_kwargs = chain_type_kwargs or {}
---> 91         combine_documents_chain = load_qa_chain(
     92             llm, chain_type=chain_type, **_chain_type_kwargs
     93         )

[c:\Users\JunXiang\AppData\Local\Programs\Python\Python38\lib\site-packages\langchain\chains\question_answering\__init__.py](file:///C:/Users/JunXiang/AppData/Local/Programs/Python/Python38/lib/site-packages/langchain/chains/question_answering/__init__.py) in load_qa_chain(llm, chain_type, verbose, callback_manager, **kwargs)
    236             f"Should be one of {loader_mapping.keys()}"
...
[c:\Users\JunXiang\AppData\Local\Programs\Python\Python38\lib\site-packages\pydantic\main.cp38-win_amd64.pyd](file:///C:/Users/JunXiang/AppData/Local/Programs/Python/Python38/lib/site-packages/pydantic/main.cp38-win_amd64.pyd) in pydantic.main.BaseModel.__init__()

ValidationError: 1 validation error for MapRerankDocumentsChain
__root__
  Output parser of llm_chain should be a RegexParser, got None (type=value_error)
```


### Expected behavior

Does not crash, when i try to run it.