[lkuffo]

Changes

• Allows for centroids to be quantized. Starting with LVQ8, and LVQ4, which seems to be enough to maintain quality in Search and Maintenance workloads.
• This PR tackles the main bottleneck of PDX IVF in large scale data: The distance form query to centroids.

Benchmarks

• LVQ4 centroids ranking have 0 effect in recall.
• LVQ4 centroids have negligible effect in maintenance.
• LVQ4 centroids are scanned 4x faster with 8x less memory usage.

Next Steps

• Explore RabitQ-4 as a faster option (with slightly more bookkeeping during maintenance)