Fix LocalDocQA object has no attribute llm when adding docs

[benjaminhuo]

Got below errors when adding docs :

AttributeError: 'LocalDocQA' object has no attribute 'llm'
2023-07-16 10:23:09.182 Uncaught app exception
Traceback (most recent call last):
  File "/Users/ben/opt/miniconda3/envs/langchain-chatglm/lib/python3.10/site-packages/streamlit/runtime/scriptrunner/script_runner.py", line 565, in _run_script
    exec(code, module.__dict__)
  File "/Users/ben/src/ai/llms/langchain-ChatGLM/webui_st.py", line 490, in <module>
    _, _, history = load_vector_store(
  File "/Users/ben/src/ai/llms/langchain-ChatGLM/webui_st.py", line 376, in load_vector_store
    return get_vector_store(
  File "/Users/ben/src/ai/llms/langchain-ChatGLM/webui_st.py", line 153, in get_vector_store
    if local_doc_qa.llm and local_doc_qa.embeddings:
AttributeError: 'LocalDocQA' object has no attribute 'llm'