remove override range_search from hnsw, use iterator-based instead

[alwayslove2013]

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[alwayslove2013]

/kind improvement

[alexanderguzhva]

@alwayslove2013 Do I get it correct that we're deprecating the range search completely?

[alwayslove2013]

@alexanderguzhva In knowhere, there are two approaches to range_search. One is an internally implemented range_search within a specific index (override, like ivf / faiss_hnsw), while the other is a unified method from the parent class (index_node) that utilizes an iterator. This iterator continuously calls next() to collect the best results that meet the specified range. https://github.com/zilliztech/knowhere/blob/8a705a0a39fecb8fb94f8edee9ab964cb0c22566/include/knowhere/index/index_node.h#L155-L171

It's a historical misalignment between milvus and knowhere. Previously, it was defined that knowhere would return all results that fit within the range. However, Milvus does not require all results and operates under a k limit (which we can name range_search_k). If the specified range is too large while range_search_k is relatively small, it can lead to significant inefficiencies. The iterator-based range_search can appropriately halt when sufficient results are collected.

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