Reduce excess matrix manipulations in item-KNN

[mdekstrand]

Speed up item-KNN by reducing some excess matrix manipulations.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.22%. Comparing base (e13381f) to head (aabef49).
Report is 15 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #678      +/-   ##
==========================================
+ Coverage   90.19%   90.22%   +0.03%     
==========================================
  Files         134      134              
  Lines        9263     9261       -2     
==========================================
+ Hits         8355     8356       +1     
+ Misses        908      905       -3     

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