Docstrings for seq2seq data

[ethanwharris]

What does this PR do?

Part of #957

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[codecov]

Codecov Report

Merging #1128 (b90561a) into master (c678582) will increase coverage by 0.17%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1128      +/-   ##
==========================================
+ Coverage   88.84%   89.02%   +0.17%     
==========================================
  Files         286      285       -1     
  Lines       12929    12961      +32     
==========================================
+ Hits        11487    11538      +51     
+ Misses       1442     1423      -19     

Flag	Coverage Δ
unittests	89.02% <100.00%> (+0.17%)	⬆️

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

Impacted Files	Coverage Δ
flash/text/__init__.py	100.00% <ø> (ø)
flash/text/seq2seq/core/__init__.py	100.00% <ø> (ø)
flash/text/seq2seq/__init__.py	100.00% <100.00%> (ø)
flash/text/seq2seq/core/input.py	97.95% <100.00%> (+12.24%)	⬆️
flash/text/seq2seq/summarization/data.py	100.00% <100.00%> (ø)
flash/text/seq2seq/translation/data.py	100.00% <100.00%> (ø)
flash/text/seq2seq/core/model.py	80.00% <0.00%> (+9.00%)	⬆️

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