[diskann-quantization] Teach PQ training to return a BasicTable.

[Mark Hildebrand (hildebrandmw)]

Now that PQ training is starting to be more diskann-quantization centric, it makes sense to return a more useful type.

[Codecov Comments Bot (codecov-commenter)]

Codecov Report

❌ Patch coverage is 92.55319% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.89%. Comparing base (a2c73bd) to head (16bed6b).

Files with missing lines	Patch %	Lines
diskann-quantization/src/product/train.rs	87.17%	5 Missing ⚠️
diskann-quantization/src/product/tables/basic.rs	92.59%	2 Missing ⚠️

Additional details and impacted files

@@                  Coverage Diff                   @@
##           mhildebr/pq-lookup    #1467      +/-   ##
======================================================
- Coverage               91.89%   90.89%   -1.01%     
======================================================
  Files                     585      586       +1     
  Lines                  116271   117202     +931     
======================================================
- Hits                   106852   106529     -323     
- Misses                   9419    10673    +1254     

Flag	Coverage Δ
miri	90.89% <92.55%> (-1.01%)	⬇️
unittests	90.58% <92.39%> (-1.26%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
diskann-benchmark/src/exhaustive/product.rs	77.88% <100.00%> (+0.07%)	⬆️
diskann-providers/src/model/pq/pq_construction.rs	93.90% <100.00%> (-0.03%)	⬇️
diskann-quantization/src/views.rs	98.82% <100.00%> (+0.05%)	⬆️
diskann-quantization/src/product/tables/basic.rs	98.40% <92.59%> (-0.98%)	⬇️
diskann-quantization/src/product/train.rs	94.39% <87.17%> (-1.81%)	⬇️

... and 57 files with indirect coverage changes

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