Add FlashDataset update

[tchaton]

What does this PR do?

Fixes #851

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[codecov]

Codecov Report

Merging #853 (54182d2) into master (5d0b329) will decrease coverage by 4.11%.
The diff coverage is 100.00%.

❗ Current head 54182d2 differs from pull request most recent head d0b9086. Consider uploading reports for the commit d0b9086 to get more accurate results

@@            Coverage Diff             @@
##           master     #853      +/-   ##
==========================================
- Coverage   89.02%   84.91%   -4.12%     
==========================================
  Files         225      225              
  Lines       12186    12187       +1     
==========================================
- Hits        10849    10349     -500     
- Misses       1337     1838     +501     

Flag	Coverage Δ
unittests	84.91% <100.00%> (-4.12%)	⬇️

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

Impacted Files	Coverage Δ
flash/core/data/datasets.py	95.71% <100.00%> (+2.96%)	⬆️
flash/text/question_answering/model.py	28.76% <0.00%> (-62.33%)	⬇️
flash/text/question_answering/data.py	27.93% <0.00%> (-59.05%)	⬇️
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/classification/data.py	55.03% <0.00%> (-37.99%)	⬇️
flash/text/seq2seq/core/utils.py	70.00% <0.00%> (-30.00%)	⬇️
flash/text/seq2seq/core/metrics.py	74.07% <0.00%> (-25.00%)	⬇️
flash/text/seq2seq/core/data.py	65.85% <0.00%> (-24.40%)	⬇️
flash/text/classification/model.py	69.09% <0.00%> (-23.64%)	⬇️
... and 6 more

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