Add ability to override the TargetFormatter in classification tasks

[ethanwharris]

What does this PR do?

Fixes #1131

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[codecov]

Codecov Report

Merging #1171 (8cc1bc4) into master (4c62482) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1171      +/-   ##
==========================================
+ Coverage   90.87%   90.88%   +0.01%     
==========================================
  Files         286      286              
  Lines       12758    12766       +8     
==========================================
+ Hits        11594    11603       +9     
+ Misses       1164     1163       -1     

Flag	Coverage Δ
unittests	90.88% <100.00%> (+0.01%)	⬆️

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

Impacted Files	Coverage Δ
flash/core/data/data_module.py	94.09% <ø> (ø)
flash/audio/classification/data.py	100.00% <100.00%> (ø)
flash/core/data/utilities/classification.py	98.24% <100.00%> (+0.02%)	⬆️
flash/image/classification/data.py	97.90% <100.00%> (+0.01%)	⬆️
flash/image/detection/data.py	100.00% <100.00%> (ø)
flash/tabular/classification/data.py	96.42% <100.00%> (+0.13%)	⬆️
flash/text/classification/data.py	100.00% <100.00%> (ø)
flash/video/classification/data.py	100.00% <100.00%> (ø)
flash/text/question_answering/model.py	92.71% <0.00%> (+0.66%)	⬆️

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