Fix backbone freezing for question answering and speech recognition

[ethanwharris]

What does this PR do?

Fixes #1112

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[codecov]

Codecov Report

Merging #1275 (890f183) into master (1d926b5) will increase coverage by 0.25%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1275      +/-   ##
==========================================
+ Coverage   91.30%   91.56%   +0.25%     
==========================================
  Files         285      284       -1     
  Lines       12779    12729      -50     
==========================================
- Hits        11668    11655      -13     
+ Misses       1111     1074      -37     

Flag	Coverage Δ
unittests	91.56% <100.00%> (+0.25%)	⬆️

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

Impacted Files	Coverage Δ
flash/audio/speech_recognition/model.py	100.00% <100.00%> (ø)
flash/text/question_answering/model.py	92.51% <100.00%> (-0.06%)	⬇️
flash/text/question_answering/collate.py	95.29% <0.00%> (-2.36%)	⬇️

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[krshrimali]

Nice fix! 🚀 Thanks, @ethanwharris! Just a couple of questions for my knowledge, but shouldn't block merging this PR.