Rename ClassificationInput to ClassificationInputMixin

[ethanwharris]

What does this PR do?

Fixes # (issue)

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[codecov]

Codecov Report

Merging #1116 (66c8e40) into master (428cdb8) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1116      +/-   ##
==========================================
- Coverage   88.50%   88.50%   -0.01%     
==========================================
  Files         286      286              
  Lines       12938    12937       -1     
==========================================
- Hits        11451    11450       -1     
  Misses       1487     1487              

Flag	Coverage Δ
unittests	88.50% <100.00%> (-0.01%)	⬇️

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

Impacted Files	Coverage Δ
flash/audio/classification/input.py	96.25% <100.00%> (ø)
flash/core/data/io/classification_input.py	100.00% <100.00%> (ø)
flash/graph/classification/input.py	89.28% <100.00%> (ø)
flash/image/classification/input.py	98.79% <100.00%> (ø)
flash/tabular/classification/input.py	92.00% <100.00%> (ø)
flash/tabular/regression/input.py	90.00% <100.00%> (ø)
flash/template/classification/data.py	95.38% <100.00%> (ø)
flash/text/classification/input.py	100.00% <100.00%> (ø)

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[tchaton]

LGTM !