Docstrings for GraphClassificationData

[ethanwharris]

What does this PR do?

Part of #957

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[codecov]

Codecov Report

Merging #1121 (f124039) into master (68c3d70) will increase coverage by 9.59%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1121      +/-   ##
==========================================
+ Coverage   79.25%   88.84%   +9.59%     
==========================================
  Files         286      286              
  Lines       12917    12931      +14     
==========================================
+ Hits        10237    11489    +1252     
+ Misses       2680     1442    -1238     

Flag	Coverage Δ
unittests	88.84% <100.00%> (+9.59%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
flash/core/data/io/classification_input.py	100.00% <100.00%> (+7.40%)	⬆️
flash/core/data/utilities/classification.py	98.19% <100.00%> (+0.01%)	⬆️
flash/core/data/utilities/samples.py	100.00% <100.00%> (+7.14%)	⬆️
flash/graph/classification/data.py	100.00% <100.00%> (ø)
flash/graph/classification/input.py	100.00% <100.00%> (+7.69%)	⬆️
flash/graph/classification/input_transform.py	100.00% <100.00%> (ø)
flash/core/data/io/input_transform.py	73.86% <0.00%> (+0.47%)	⬆️
flash/image/classification/data.py	97.58% <0.00%> (+0.80%)	⬆️
flash/image/segmentation/model.py	92.50% <0.00%> (+1.25%)	⬆️
flash/image/classification/model.py	78.84% <0.00%> (+1.92%)	⬆️
... and 63 more

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