Add sampler argument to tabular data

[ethanwharris]

What does this PR do?

Fixes # (issue)

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[codecov]

Codecov Report

Merging #792 (b510f52) into master (ca3870a) will decrease coverage by 4.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #792      +/-   ##
==========================================
- Coverage   89.83%   85.83%   -4.01%     
==========================================
  Files         213      213              
  Lines       11404    11405       +1     
==========================================
- Hits        10245     9789     -456     
- Misses       1159     1616     +457     

Flag	Coverage Δ
unittests	85.83% <100.00%> (-4.01%)	⬇️

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

Impacted Files	Coverage Δ
flash/tabular/data.py	93.71% <100.00%> (+0.03%)	⬆️
flash/text/question_answering/model.py	29.16% <0.00%> (-61.81%)	⬇️
flash/text/question_answering/data.py	28.29% <0.00%> (-58.53%)	⬇️
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	57.84% <0.00%> (-31.40%)	⬇️
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 4 more

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[tchaton]

LGTM !