Redesign graph datasets

[CarloLucibello]

Part of #73

[codecov-commenter]

Codecov Report

Merging #114 (c64cb00) into master (7aae492) will decrease coverage by 2.52%.
The diff coverage is 57.51%.

@@            Coverage Diff             @@
##           master     #114      +/-   ##
==========================================
- Coverage   37.98%   35.45%   -2.53%     
==========================================
  Files          36       37       +1     
  Lines        1290     1365      +75     
==========================================
- Hits          490      484       -6     
- Misses        800      881      +81     

Impacted Files	Coverage Δ
src/abstract_datasets.jl	25.00% <0.00%> (ø)
src/datasets/graphs/TUDataset/TUDataset.jl	3.63% <ø> (ø)
src/datasets/graphs/ogbdataset.jl	1.42% <0.00%> (ø)
src/datasets/misc/iris.jl	61.29% <ø> (ø)
src/datasets/misc/mutagenesis.jl	53.19% <ø> (ø)
src/datasets/text/ptblm.jl	6.25% <ø> (ø)
src/datasets/text/smsspamcollection.jl	51.85% <ø> (ø)
src/datasets/text/udenglish.jl	56.86% <ø> (ø)
src/datasets/vision/cifar10.jl	2.50% <ø> (ø)
src/datasets/vision/cifar100.jl	2.46% <ø> (ø)
... and 22 more

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