Chainer graph refactor

[corochann]

Support automatic data download. Please try running code under examples/network_graph

python train_network_graph.py --dataset cora --method gin --device 0
python train_network_graph.py --dataset citeseer --method gin_sparse --device 0
python train_network_graph.py --dataset reddit --device 0 --method gin --coo true

[codecov-io]

Codecov Report

Merging #402 into master will decrease coverage by 4.49%. The diff coverage is n/a.

@@            Coverage Diff            @@
##           master     #402     +/-   ##
=========================================
- Coverage   88.91%   84.41%   -4.5%     
=========================================
  Files         234      234             
  Lines       11449    11449             
=========================================
- Hits        10180     9665    -515     
- Misses       1269     1784    +515

[corochann]

/test

[pfn-ci-bot]

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