Cleans up code like bolts

[oke-aditya]

Following PR in bolts.
It was a nice refactor to clean code. I moved the torchvision files to _model_zoo.py !
Now we can easily create a model zoo.

Thanks to @Borda and @akihironitta !!

[codecov]

Codecov Report

Merging #65 (66d7cf3) into master (e7b075e) will decrease coverage by 0.12%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #65      +/-   ##
==========================================
- Coverage   78.78%   78.66%   -0.13%     
==========================================
  Files          41       42       +1     
  Lines        1537     1514      -23     
==========================================
- Hits         1211     1191      -20     
+ Misses        326      323       -3     

Flag	Coverage Δ
unittests	78.66% <100.00%> (-0.13%)	⬇️

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Impacted Files	Coverage Δ
...kvision/models/components/torchvision_backbones.py	96.82% <100.00%> (+2.50%)	⬆️
quickvision/pretrained/_model_zoo.py	100.00% <100.00%> (ø)
quickvision/pretrained/_pretrained_cnns.py	100.00% <100.00%> (ø)

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