Remove indexPath when computing hash value used for naming exact NN file

[huynmg]

#391

Exact NN should be independent of the indexPath as we should remove it when computing hash value used for naming exact NN File. This will help reduce some extra computation when the indexPath changes but the exact NNs doesn't change .e.g. when benchmarking different quantization level for the same data.

Re-running the benchmarks before & after the change:

Before (exact NN are computed 2 times as indexPath is part of the hash computation)

recall  latency(ms)    nDoc  topK  fanout  maxConn  beamWidth  quantized  index(s)  index_docs/s  num_segments  index_size(MB)  vec_disk(MB)  vec_RAM(MB)  indexType
 0.832        0.630  100000   100      50       64        250     4 bits     16.05       6231.31             1          116.66       110.245       12.589       HNSW
 0.898        0.810  100000   100      50       64        250     7 bits     22.29       4487.32             1          129.09       122.452       24.796       HNSW

After (exact NN is reused)

recall  latency(ms)    nDoc  topK  fanout  maxConn  beamWidth  quantized  index(s)  index_docs/s  num_segments  index_size(MB)  vec_disk(MB)  vec_RAM(MB)  indexType
 0.837        0.540  100000   100      50       64        250     4 bits     17.96       5567.62             1          116.68       110.245       12.589       HNSW
 0.897        0.800  100000   100      50       64        250     7 bits     25.18       3971.09             1          128.89       122.452       24.796       HNSW

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