Add langchain chatbot

[YanxinLu]

No description provided.

[research4pan]

Thanks very much! Some changes and questions regarding the PR.

README.md

• [Style] Better add a runnable example with example files, along with its download script in data/download.sh

contrib/langchain/retrieval_chatbot.py

• [Question] line 73, 81: I am wondering if splitting documents by a fixed number of char size without overlap is the common approach, since there may be losses near the boundary.
• [Style] line 173: Better make this chat history file path an argument with a default value of tmp/chat_history.
• [Style] line 174: 'abc123' better to be other interpretable keywords, or a constant variable with interpretable names.

[research4pan]

LGTM, thanks 👍 Some minor points that can improved later

contrib/langchain/README.md

• [Style] We normally use naming convention like --set_url instead of --set-url.
• [Style] We can add separated example usages for URL retrieval and text retrieval.