ConversationalRetrievalChain having trouble with custom 'combine_docs_chain'
Labels: name: auto:bug, name: area: models

### System Info

Langchain 0.247
Python 3.10.11

### Who can help?

@chase

### Information

- [ ] The official example notebooks/scripts
- [X] My own modified scripts

### Related Components

- [X] LLMs/Chat Models
- [ ] Embedding Models
- [X] Prompts / Prompt Templates / Prompt Selectors
- [ ] Output Parsers
- [ ] Document Loaders
- [ ] Vector Stores / Retrievers
- [ ] Memory
- [ ] Agents / Agent Executors
- [ ] Tools / Toolkits
- [ ] Chains
- [ ] Callbacks/Tracing
- [ ] Async

### Reproduction

I wanted to adapt the original combine_docs_chain to some customization where a user input (e.g. 'you act like a HR chatbot') would be added to the original prompt. The workaround I found was to rewrite the original prompt files to take in a user input, which works. However, I'm having trouble making the ConversationRetrievalChain work.

1. Adapt the original templates from the source files and load them in:
I wanted to add the possibility to add some user input to the pre-made prompts from langchain.
So I duplicated the original files and adapted the text of the prompts somewhat
```
from docs.stuff_prompt import create_stuff_prompt_selector
STUFF_PROMPT_SELECTOR = create_stuff_prompt_selector(ui_input= ui_input) #adds the ui_input to the prompt
stuff_prompt = STUFF_PROMPT_SELECTOR.get_prompt(llm) #based on LLM model, it will return either the regular PromptTemplate version or the chat (ChatPromptTemplate). Same working principle as in the source files
combine_docs_chain = load_qa_chain(llm = llm,
          chain_type = 'stuff',
          prompt = stuff_prompt
          ) #create a custom combine_docs_chain
```
2. Create the ConversationalRetrievalChain.from_llm() object with the custom combine_docs_chain
Simply create the object. First time might work, but second won't work
```
from langchain.chains import ConversationalRetrievalChain
qa = ConversationalRetrievalChain.from_llm(
    llm,
    retriever=vectordb.as_retriever(),
    return_source_documents = True,
    memory=memory,
    verbose = True,
    combine_docs_chain = combine_docs_chain
)
```
3. This yields an error:
```
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Cell In[25], line 2
      1 from langchain.chains import ConversationalRetrievalChain
----> 2 qa = ConversationalRetrievalChain.from_llm(
      3     llm,
      4     retriever=vectordb.as_retriever(),
      5     return_source_documents = True,        # when return_source_documents is True, a workaround needs to be found https://github.com/langchain-ai/langchain/issues/2303
      6     memory=memory,
      7     #, get_chat_history=lambda h : h
      8     verbose = True,
      9     combine_docs_chain = combine_docs_chain
     10     #question_generator
     11 )

File [c:\Users\XXX\Anaconda3\envs\langchain-env\lib\site-packages\langchain\chains\conversational_retrieval\base.py:357](file:///C:/Users/XXX/Anaconda3/envs/langchain-env/lib/site-packages/langchain/chains/conversational_retrieval/base.py:357), in ConversationalRetrievalChain.from_llm(cls, llm, retriever, condense_question_prompt, chain_type, verbose, condense_question_llm, combine_docs_chain_kwargs, callbacks, **kwargs)
    350 _llm = condense_question_llm or llm
    351 condense_question_chain = LLMChain(
    352     llm=_llm,
    353     prompt=condense_question_prompt,
    354     verbose=verbose,
    355     callbacks=callbacks,
    356 )
--> 357 return cls(
    358     retriever=retriever,
    359     combine_docs_chain=doc_chain,
    360     question_generator=condense_question_chain,
    361     callbacks=callbacks,
    362     **kwargs,
    363 )

TypeError: langchain.chains.conversational_retrieval.base.ConversationalRetrievalChain() got multiple values for keyword argument 'combine_docs_chain'
```

### Expected behavior

It is expected to take in the combine_docs_chain I made and initiate the ConversationalRetrievalChain. Is this a cache error? Where the ConversationalRetrievalChain cannot be changed after it has been initiated before?