Added the reduction op input parameter to host_scalar_(all)reduce utility functions.

[seunghwak]

Currently, host_scalar_(all)reduce assumes SUM reduction. Adding this input parameter will be necessary for other types of reductions (e.g. MAX).

[codecov-commenter]

Codecov Report

Merging #1822 (3c5bb2d) into branch-21.10 (bf64c2c) will increase coverage by 9.72%.
The diff coverage is n/a.

@@               Coverage Diff                @@
##           branch-21.10    #1822      +/-   ##
================================================
+ Coverage         59.85%   69.57%   +9.72%     
================================================
  Files                77      139      +62     
  Lines              3547     8645    +5098     
================================================
+ Hits               2123     6015    +3892     
- Misses             1424     2630    +1206     

Impacted Files	Coverage Δ
python/cugraph/layout/__init__.py
python/cugraph/dask/common/mg_utils.py
python/cugraph/utilities/utils.py
python/cugraph/dask/community/louvain.py
python/cugraph/dask/traversal/bfs.py
python/cugraph/community/subgraph_extraction.py
python/cugraph/sampling/random_walks.py
python/cugraph/community/louvain.py
python/cugraph/community/triangle_count.py
python/cugraph/structure/shuffle.py
... and 206 more

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[seunghwak]

rerun tests

[ChuckHastings]

rerun tests

[ChuckHastings]

rerun tests

[ChuckHastings]

@gpucibot merge