Parallelize training in PQVIF

[habedi]

• Parallelized the training phase in the PQVIF index implementation.
• Parallelized BulkAdd in the HNSW implementation.
• Fixed a few bugs.

[codecov]

Codecov Report

❌ Patch coverage is 83.17757% with 54 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.44%. Comparing base (ea27263) to head (181cd14).

Files with missing lines	Patch %	Lines
pqivf/index.go	80.58%	22 Missing and 11 partials ⚠️
hnsw/index.go	86.09%	14 Missing and 7 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #23      +/-   ##
==========================================
- Coverage   86.75%   86.44%   -0.32%     
==========================================
  Files          13       13              
  Lines        2960     3172     +212     
==========================================
+ Hits         2568     2742     +174     
- Misses        268      294      +26     
- Partials      124      136      +12     

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