Move the utility function to gather distributed vectors from tests/community to tests/utilities and update MG tests

[seunghwak]

• Move the utility function to gather distributed vectors from tests/community/mg_louvain_helper.cu to tests/utilities/device_comm_wrapper.cu and rename the function to device_gatherv
• Update MG tests to gather the MG results in root and compare with the SG result in root.
• Few more minor updates (sort_by_key thrust wrapper return value, adding missing const in input parameter, add missing include statement, add barrier in MG performance measurement)

This update is necessary to simplify MG WCC testing.

[codecov-commenter]

Codecov Report

Merging #1602 (38af997) into branch-0.20 (250f2c8) will decrease coverage by 1.93%.
The diff coverage is 62.76%.

❗ Current head 38af997 differs from pull request most recent head 1759714. Consider uploading reports for the commit 1759714 to get more accurate results

@@               Coverage Diff               @@
##           branch-0.20    #1602      +/-   ##
===============================================
- Coverage        61.04%   59.10%   -1.94%     
===============================================
  Files               73       79       +6     
  Lines             3219     3482     +263     
===============================================
+ Hits              1965     2058      +93     
- Misses            1254     1424     +170     

Impacted Files	Coverage Δ
python/cugraph/__init__.py	100.00% <ø> (ø)
python/cugraph/components/connectivity.py	95.65% <ø> (ø)
python/cugraph/dask/centrality/katz_centrality.py	29.16% <ø> (ø)
python/cugraph/dask/link_analysis/pagerank.py	21.87% <0.00%> (ø)
python/cugraph/dask/traversal/bfs.py	25.00% <0.00%> (-2.59%)	⬇️
python/cugraph/traversal/ms_bfs.py	11.11% <ø> (ø)
...ure/graph_implementation/simpleDistributedGraph.py	22.72% <22.72%> (ø)
python/cugraph/generators/rmat.py	34.44% <34.44%> (ø)
python/cugraph/linear_assignment/lap.py	71.42% <50.00%> (-10.39%)	⬇️
python/cugraph/structure/number_map.py	63.63% <57.14%> (-0.19%)	⬇️
... and 43 more

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[BradReesWork]

@gpucibot merge