Clean-up to fix rtd build

[ethanwharris]

What does this PR do?

RTD mocks requirements when it builds the docs. This includes the PL requirement so the LightningEnum gets switched to just a regular type and so doesn't support iteration. The cleans up a bit and avoids the iteration.

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[codecov]

Codecov Report

Merging #1259 (97a6d71) into master (2dd4903) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1259   +/-   ##
=======================================
  Coverage   91.17%   91.18%           
=======================================
  Files         286      286           
  Lines       12788    12788           
=======================================
+ Hits        11660    11661    +1     
+ Misses       1128     1127    -1     

Flag	Coverage Δ
unittests	91.18% <100.00%> (+<0.01%)	⬆️

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

Impacted Files	Coverage Δ
flash/core/finetuning.py	92.20% <100.00%> (ø)
flash/text/question_answering/model.py	92.56% <0.00%> (+0.67%)	⬆️

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