Enabled C++ memory-map usage in PECOS-HNSW

[OctoberChang]

Issue #, if available:

Description of changes:

• Enabled memory-map usage for C++ interface of PECOS-HNSW, which supports faster loading of the HNSW index.
• Fixed potential overview issues when offsetting memory pointer of PQ codebooks in HNSW-PQ4.

An C++ example code:

// train and save into memory-mapped format
pecos::ann::HNSW<float, feat_vec_t> indexer_;
indexer_.train(X_trn, M, efC, threads);
indexer_.save(output_dir);

// load from mmap-store, with lazy_load=false
pecos::ann::HNSW<float, feat_vec_t> indexer;
bool lazy_load = false;
indexer.load(output_dir , lazy_load);

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[rofuyu]

We need to fix all "Multiplication result converted to larger type" warning properly.