Docstrings for tabular data

[ethanwharris]

What does this PR do?

Part of #957

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[codecov]

Codecov Report

Merging #1115 (403680a) into master (1bbd7d7) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1115      +/-   ##
==========================================
+ Coverage   88.53%   88.55%   +0.01%     
==========================================
  Files         286      286              
  Lines       12950    12954       +4     
==========================================
+ Hits        11465    11471       +6     
+ Misses       1485     1483       -2     

Flag	Coverage Δ
unittests	88.55% <100.00%> (+0.01%)	⬆️

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

Impacted Files	Coverage Δ
flash/tabular/classification/data.py	96.15% <100.00%> (+0.32%)	⬆️
flash/tabular/regression/data.py	96.15% <100.00%> (+0.32%)	⬆️
flash/tabular/classification/input.py	96.00% <0.00%> (+4.00%)	⬆️
flash/tabular/regression/input.py	95.00% <0.00%> (+5.00%)	⬆️

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