Fix RTD build

[ethanwharris]

What does this PR do?

Fixes # (issue)

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[codecov]

Codecov Report

Merging #789 (75cd1c6) into master (ae525d9) will decrease coverage by 3.86%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #789      +/-   ##
==========================================
- Coverage   89.61%   85.75%   -3.87%     
==========================================
  Files         213      213              
  Lines       11309    11309              
==========================================
- Hits        10135     9698     -437     
- Misses       1174     1611     +437     

Flag	Coverage Δ
unittests	85.75% <ø> (-3.87%)	⬇️

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

Impacted Files	Coverage Δ
flash/core/adapter.py	94.44% <ø> (ø)
flash/core/model.py	87.58% <ø> (ø)
flash/image/classification/adapters.py	82.02% <ø> (ø)
flash/text/question_answering/model.py	29.16% <0.00%> (-61.81%)	⬇️
flash/text/question_answering/data.py	28.29% <0.00%> (-58.53%)	⬇️
flash/text/classification/cli.py	53.84% <0.00%> (-46.16%)	⬇️
flash/text/question_answering/cli.py	60.00% <0.00%> (-40.00%)	⬇️
flash/text/seq2seq/core/utils.py	70.00% <0.00%> (-30.00%)	⬇️
flash/text/classification/data.py	62.92% <0.00%> (-28.09%)	⬇️
flash/text/seq2seq/core/metrics.py	74.07% <0.00%> (-25.00%)	⬇️
... and 6 more

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[tchaton]

LGTM !