Reformat text classification data

[pietrolesci]

What does this PR do?

Reformats the text classification data.

[codecov]

Codecov Report

Merging #998 (907ae1e) into master (4cda031) will decrease coverage by 3.37%.
The diff coverage is 13.79%.

@@            Coverage Diff             @@
##           master     #998      +/-   ##
==========================================
- Coverage   88.40%   85.02%   -3.38%     
==========================================
  Files         250      250              
  Lines       13199    13199              
==========================================
- Hits        11668    11222     -446     
- Misses       1531     1977     +446     

Flag	Coverage Δ
unittests	85.02% <13.79%> (-3.38%)	⬇️

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

Impacted Files	Coverage Δ
flash/text/classification/data.py	56.47% <13.79%> (-40.42%)	⬇️
flash/text/question_answering/model.py	28.27% <0.00%> (-62.76%)	⬇️
flash/text/question_answering/data.py	28.16% <0.00%> (-58.87%)	⬇️
flash/text/classification/cli.py	53.84% <0.00%> (-46.16%)	⬇️
flash/text/question_answering/cli.py	60.00% <0.00%> (-40.00%)	⬇️
flash/text/seq2seq/core/data.py	66.06% <0.00%> (-24.25%)	⬇️
flash/text/question_answering/finetuning.py	3.92% <0.00%> (-19.61%)	⬇️
flash/text/classification/model.py	77.55% <0.00%> (-14.29%)	⬇️
flash/text/seq2seq/core/model.py	60.00% <0.00%> (-8.89%)	⬇️
... and 5 more

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