[sparse search] Add sparse search agent

[w5688414]

• query expansion (few shot for hard examples)
• query repetition
• reciprocal rank fusion

    retriever = CustomSearch(base_url=base_url, outId=outId, key=key)
    examples = [
        {"query": "半监督的论文有哪些？", "data": {"keywords": ["半监督"]}},
        {"query": "请推荐5篇半监督学习的论文", "data": {"keywords": ["半监督学习"], "pageSize": 5}},
    ]
    agent = SparseSearchAgent(llm, retriever, few_shots=examples)

    # query = '"计算机科学" 与 "生物学" 交叉学科论文'
    # query = '影响力最高的 "心理学" 研究论文'
    query = "请推荐5篇半监督学习的论文"
    docs = asyncio.run(agent.run(query))
    for idx, item in enumerate(docs):
        print(str(idx) + ".", item["title"], item["keyword"], item["year"])
    print(len(docs))