fix: fp16 GPU training

[chebbyChefNEQ]

• cast ivf centroids to the same type as vectors before inference
• cast codebook to the same type as ivf centroid before passing to rust
• fix typing syntax

[eddyxu]

could you just add one small test

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 77.05%. Comparing base (8155c25) to head (f1b6048).

Files with missing lines	Patch %	Lines
rust/lance-index/src/vector/ivf/shuffler.rs	0.00%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3105      +/-   ##
==========================================
+ Coverage   77.03%   77.05%   +0.02%     
==========================================
  Files         240      240              
  Lines       80410    80412       +2     
  Branches    80410    80412       +2     
==========================================
+ Hits        61943    61965      +22     
+ Misses      15265    15263       -2     
+ Partials     3202     3184      -18     

Flag	Coverage Δ
unittests	77.05% <0.00%> (+0.02%)	⬆️

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[eddyxu]

LGTM