Fix ReduceLROnPlateau

[ethanwharris]

What does this PR do?

Fixes #1244

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[codecov]

Codecov Report

Merging #1251 (f802d8e) into master (64e7589) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1251      +/-   ##
==========================================
- Coverage   91.19%   91.18%   -0.01%     
==========================================
  Files         286      286              
  Lines       12793    12791       -2     
==========================================
- Hits        11666    11663       -3     
- Misses       1127     1128       +1     

Flag	Coverage Δ
unittests	91.18% <ø> (-0.01%)	⬇️

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

Impacted Files	Coverage Δ
flash/core/model.py	87.71% <ø> (-0.07%)	⬇️
flash/text/question_answering/model.py	91.89% <0.00%> (-0.68%)	⬇️

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[krshrimali]

Really nice doctests! Thanks for the fix, @ethanwharris 🚀 LGTM!