Refactor tabular data to use classification targets handling

[ethanwharris]

What does this PR do?

Refactors tabular classification data to use the classification targets handling. Also includes some other fixes.

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[codecov]

Codecov Report

Merging #1114 (b5ae628) into master (127b7c0) will decrease coverage by 0.92%.
The diff coverage is 93.61%.

@@            Coverage Diff             @@
##           master    #1114      +/-   ##
==========================================
- Coverage   88.49%   87.57%   -0.93%     
==========================================
  Files         284      286       +2     
  Lines       12891    12938      +47     
==========================================
- Hits        11408    11330      -78     
- Misses       1483     1608     +125     

Flag	Coverage Δ
unittests	87.57% <93.61%> (-0.93%)	⬇️

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

Impacted Files	Coverage Δ
flash/tabular/classification/cli.py	100.00% <ø> (ø)
flash/tabular/classification/model.py	96.66% <ø> (ø)
flash/tabular/classification/utils.py	97.29% <ø> (-0.14%)	⬇️
flash/tabular/regression/cli.py	100.00% <ø> (ø)
flash/tabular/regression/model.py	100.00% <ø> (ø)
flash/tabular/regression/input.py	90.00% <90.00%> (ø)
flash/tabular/classification/input.py	92.00% <92.00%> (ø)
flash/tabular/classification/data.py	95.83% <95.45%> (-4.17%)	⬇️
flash/tabular/regression/data.py	95.83% <95.45%> (-4.17%)	⬇️
flash/tabular/data.py	100.00% <100.00%> (+1.88%)	⬆️
... and 20 more

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