Multi-label text classification

[ethanwharris]

What does this PR do?

Fixes #397 Fixes #396

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[codecov]

Codecov Report

Merging #401 (1101871) into master (bec7142) will increase coverage by 4.14%.
The diff coverage is 80.00%.

@@            Coverage Diff             @@
##           master     #401      +/-   ##
==========================================
+ Coverage   80.56%   84.71%   +4.14%     
==========================================
  Files         109      109              
  Lines        6591     6607      +16     
==========================================
+ Hits         5310     5597     +287     
+ Misses       1281     1010     -271     

Flag	Coverage Δ
unittests	84.71% <80.00%> (+4.14%)	⬆️

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

Impacted Files	Coverage Δ
flash/text/classification/data.py	84.21% <72.72%> (+47.14%)	⬆️
flash/text/classification/model.py	88.63% <77.77%> (+49.61%)	⬆️
flash/core/data/data_module.py	93.54% <100.00%> (+0.49%)	⬆️
flash/core/model.py	80.84% <100.00%> (+6.12%)	⬆️
flash/core/data/properties.py	100.00% <0.00%> (+1.33%)	⬆️
flash/text/seq2seq/translation/data.py	100.00% <0.00%> (+12.50%)	⬆️
flash/text/seq2seq/summarization/data.py	100.00% <0.00%> (+12.50%)	⬆️
... and 9 more

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[tchaton]

LGTM !