refactor: use transposed centroids in PQ preprocess

[usamoi]

With this PR, on GIST, (nlist = 4096, residual_quantization = true)

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nprobe = 300

• IVF-RQ 0.9843 2.5s; 1x memory usage
• IVF-PQ 0.9914 3.3s (ratio = 2, bits = 4; pq_rerank_size = 100); 2x memory usage
• IVF-PQ 0.9812 2.7s (ratio = 3, bits = 4; pq_rerank_size = 188); 1.5x memory usage

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nprobe = 200

• IVF-RQ 0.9720 1.9s; 1x memory usage
• IVF-PQ 0.9731 2.3s (ratio = 2, bits = 4; pq_rerank_size = 100); 2x memory usage
• IVF-PQ 0.9696 2.0s (ratio = 3, bits = 4; pq_rerank_size = 188); 1.5x memory usage

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nprobe = 100

• IVF-RQ 0.9258 1.3s; 1x memory usage
• IVF-PQ 0.9250 1.5s (ratio = 2, bits = 4; pq_rerank_size = 100); 2x memory usage
• IVF-PQ 0.9276 1.3s (ratio = 3, bits = 4; pq_rerank_size = 188); 1.5x memory usage

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