Removed NetworkX requirement for type checks, fixed docstring, added new docstrings, import cleanups

[rlratzel]

• Changed type check code to no longer require NetworkX and instead only check Nx types if installed. [BUG] Pagerank requires NetworkX #1851
• Cleaned up imports, removed unused imports.
• Changed import_optional to use importlib instead, and return MissingModule instances for easier error reporting.
• Fixed format of example in rmat docstring [DOC] RMAT documentation example is badly formatted. #1807
• Added docstrings for pylibcugraph *_connected_components APIs. [FEA] initial pylibcugraph support #1782

Tested changes to optional import and type checking code by uninstalling NetworkX, observing the failure shown in #1851, applying change, then testing to observe the same script runs to completion without Nx installed.

closes #1851
closes #1807
closes #1782

[codecov-commenter]

Codecov Report

Merging #1853 (b786f01) into branch-21.10 (177e9ad) will increase coverage by 0.49%.
The diff coverage is 86.00%.

❗ Current head b786f01 differs from pull request most recent head 42c536d. Consider uploading reports for the commit 42c536d to get more accurate results

@@               Coverage Diff                @@
##           branch-21.10    #1853      +/-   ##
================================================
+ Coverage         69.59%   70.08%   +0.49%     
================================================
  Files               143      143              
  Lines              8675     8819     +144     
================================================
+ Hits               6037     6181     +144     
  Misses             2638     2638              

Impacted Files	Coverage Δ
python/cugraph/cugraph/generators/rmat.py	34.44% <ø> (ø)
python/cugraph/cugraph/structure/graph_classes.py	76.84% <0.00%> (+0.55%)	⬆️
python/cugraph/cugraph/utilities/__init__.py	100.00% <ø> (ø)
python/cugraph/cugraph/link_prediction/overlap.py	54.54% <50.00%> (-3.79%)	⬇️
python/cugraph/cugraph/link_prediction/sorensen.py	62.96% <50.00%> (-2.56%)	⬇️
python/cugraph/cugraph/utilities/utils.py	70.18% <75.00%> (+1.39%)	⬆️
...graph/cugraph/centrality/betweenness_centrality.py	90.16% <100.00%> (+0.16%)	⬆️
...thon/cugraph/cugraph/centrality/katz_centrality.py	89.47% <100.00%> (ø)
python/cugraph/cugraph/community/ecg.py	100.00% <100.00%> (ø)
python/cugraph/cugraph/community/egonet.py	92.10% <100.00%> (-0.21%)	⬇️
... and 78 more

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[jnke2016]

Looks good

[BradReesWork]

@gpucibot merge