HotFix for doc build on master

[SeanNaren]

What does this PR do?

Currently the docs are failing on master. This seems to be because of a numpy error. Mocking this seems to not work, thus for now installing the library within the docs may be a suitable hotfix.

[codecov]

Codecov Report

Merging #849 (a57dd98) into master (eb1fcd3) will decrease coverage by 11.15%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master     #849       +/-   ##
===========================================
- Coverage   89.00%   77.84%   -11.16%     
===========================================
  Files         224      224               
  Lines       12117    12117               
===========================================
- Hits        10785     9433     -1352     
- Misses       1332     2684     +1352     

Flag	Coverage Δ
unittests	77.84% <ø> (-11.16%)	⬇️

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Impacted Files	Coverage Δ
flash/core/integrations/icevision/transforms.py	13.33% <0.00%> (-80.84%)	⬇️
...lash/image/embedding/vissl/transforms/multicrop.py	27.27% <0.00%> (-69.70%)	⬇️
...lash/image/embedding/vissl/transforms/utilities.py	14.63% <0.00%> (-68.30%)	⬇️
flash/image/detection/serialization.py	28.57% <0.00%> (-65.31%)	⬇️
flash/image/embedding/heads/vissl_heads.py	36.36% <0.00%> (-63.64%)	⬇️
flash/text/question_answering/model.py	28.76% <0.00%> (-62.33%)	⬇️
flash/core/integrations/icevision/backbones.py	33.33% <0.00%> (-60.01%)	⬇️
flash/text/question_answering/data.py	27.93% <0.00%> (-59.05%)	⬇️
...age/face_detection/backbones/fastface_backbones.py	42.10% <0.00%> (-57.90%)	⬇️
flash/image/embedding/vissl/adapter.py	31.76% <0.00%> (-56.48%)	⬇️
... and 48 more

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