the faiss index method change: replace the IndexFlatIP with IndexIVFFlat

[Adeshen]

IndexFlatIP is brute searching method. IndexIVFFlat use aggregation to lower the dimension. So it will be faster, but lower the precision.

[tpoisonooo]

IndexIVFFlat needs clustering and L2 norm as distance method, see https://zhuanlan.zhihu.com/p/40236865

IndexIVFFlat may cause precision drop.

[tpoisonooo]

See also https://github.com/facebookresearch/faiss/wiki/Faiss-indexes