Add option for nested tasks

[ethanwharris]

What does this PR do?

Fixes # (issue)

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[codecov]

Codecov Report

Merging #575 (a404f29) into master (fc3263f) will increase coverage by 1.55%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #575      +/-   ##
==========================================
+ Coverage   89.86%   91.41%   +1.55%     
==========================================
  Files         117      117              
  Lines        7190     7200      +10     
==========================================
+ Hits         6461     6582     +121     
+ Misses        729      618     -111     

Flag	Coverage Δ
unittests	91.41% <100.00%> (+1.55%)	⬆️

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

Impacted Files	Coverage Δ
flash/core/model.py	86.52% <100.00%> (+0.35%)	⬆️
flash/core/data/data_module.py	93.53% <0.00%> (+0.43%)	⬆️
flash/text/seq2seq/translation/model.py	76.19% <0.00%> (+4.76%)	⬆️
flash/text/seq2seq/summarization/model.py	80.00% <0.00%> (+5.00%)	⬆️
flash/text/seq2seq/core/model.py	75.36% <0.00%> (+8.69%)	⬆️
flash/text/seq2seq/core/data.py	92.00% <0.00%> (+20.00%)	⬆️
flash/text/classification/data.py	89.09% <0.00%> (+22.42%)	⬆️
flash/text/classification/model.py	93.18% <0.00%> (+22.72%)	⬆️
flash/text/seq2seq/core/metrics.py	97.22% <0.00%> (+23.14%)	⬆️

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