Fix summarization postprocess backbone

[ethanwharris]

What does this PR do?

Fixes # (issue)

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[codecov]

Codecov Report

Merging #296 (5706bf7) into master (1f50b3f) will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #296      +/-   ##
==========================================
+ Coverage   88.44%   88.50%   +0.05%     
==========================================
  Files          74       74              
  Lines        3626     3644      +18     
==========================================
+ Hits         3207     3225      +18     
  Misses        419      419              

Flag	Coverage Δ
unittests	88.50% <100.00%> (+0.05%)	⬆️

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

Impacted Files	Coverage Δ
flash/text/seq2seq/core/data.py	88.46% <100.00%> (+0.96%)	⬆️
flash/text/seq2seq/summarization/data.py	88.88% <100.00%> (+8.88%)	⬆️

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[tchaton]

Nice !