feat: Added PyTorch support of VGG backbones

[fg-mindee]

Following up on #261, this PR introduces the following modifications:

• added pytorch version of vgg16_bn
• updated unittests

Any feedback is welcome!

[codecov]

Codecov Report

Merging #328 (1ccc0cb) into main (018d44a) will decrease coverage by 3.14%.
The diff coverage is 93.47%.

@@            Coverage Diff             @@
##             main     #328      +/-   ##
==========================================
- Coverage   97.61%   94.47%   -3.15%     
==========================================
  Files          32       74      +42     
  Lines        1007     2913    +1906     
==========================================
+ Hits          983     2752    +1769     
- Misses         24      161     +137     

Flag	Coverage Δ
unittests	94.47% <93.47%> (-3.15%)	⬇️

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

Impacted Files	Coverage Δ
doctr/models/recognition/master.py	69.56% <69.56%> (ø)
doctr/file_utils.py	74.46% <74.46%> (ø)
doctr/models/recognition/transformer.py	87.09% <87.09%> (ø)
doctr/models/detection/linknet/base.py	87.67% <87.67%> (ø)
doctr/models/data_utils.py	88.67% <88.67%> (ø)
doctr/datasets/recognition.py	89.47% <89.47%> (ø)
doctr/transforms/modules/base.py	89.65% <89.65%> (ø)
doctr/utils/visualization.py	93.02% <91.30%> (-6.98%)	⬇️
doctr/datasets/ocr.py	91.66% <91.66%> (ø)
...dels/detection/differentiable_binarization/base.py	91.71% <91.71%> (ø)
... and 105 more

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[charlesmindee]

Thanks!