Make it possible to customize the CuIterator adaptor.

[maleadt]

cc @DhairyaLGandhi

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 72.07%. Comparing base (0044ea2) to head (1143ee8).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2297      +/-   ##
==========================================
+ Coverage   72.06%   72.07%   +0.01%     
==========================================
  Files         155      155              
  Lines       14881    14884       +3     
==========================================
+ Hits        10724    10728       +4     
+ Misses       4157     4156       -1     

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