Resolve inconsistencies in reduction support in primitives

[seunghwak]

Resolve inconsistencies in reduction support in primitives (partially addresses #2003).

• If reduce_op is missing, + is assumed.
• If init is missing, T{} is assumed.
• Provides a pre-defined set of reduction operations in reduce_op.cuh
• Provides a guidance on writing custom reduction operators.
• Fix a bug in reduction support for thrust::tuple (std::numeric_limits does not work properly for thrust::tuple, std::numeric_limits should be applied to individual elements of thrust::tuple).

[codecov-commenter]

Codecov Report

Merging #2257 (9dee12b) into branch-22.06 (34a40a7) will not change coverage.
The diff coverage is n/a.

❗ Current head 9dee12b differs from pull request most recent head b2f151a. Consider uploading reports for the commit b2f151a to get more accurate results

@@              Coverage Diff              @@
##           branch-22.06    #2257   +/-   ##
=============================================
  Coverage         71.18%   71.18%           
=============================================
  Files               172      172           
  Lines             11277    11277           
=============================================
  Hits               8027     8027           
  Misses             3250     3250           

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[ChuckHastings]

@gpucibot merge