Added MMDetection VFNet Support.

[ai-fast-track]

I added MMDetection VFNet Support.

Paper: https://arxiv.org/pdf/2008.13367.pdf

closes #905

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Codecov Report

Merging #906 (be93f99) into master (8739c4a) will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #906      +/-   ##
==========================================
+ Coverage   87.45%   87.51%   +0.06%     
==========================================
  Files         232      235       +3     
  Lines        5015     5040      +25     
==========================================
+ Hits         4386     4411      +25     
  Misses        629      629              

Flag	Coverage Δ
unittests	87.51% <100.00%> (+0.06%)	⬆️

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Impacted Files	Coverage Δ
icevision/models/mmdet/models/__init__.py	100.00% <100.00%> (ø)
icevision/models/mmdet/models/vfnet/__init__.py	100.00% <100.00%> (ø)
...on/models/mmdet/models/vfnet/backbones/__init__.py	100.00% <100.00%> (ø)
.../models/mmdet/models/vfnet/backbones/resnet_fpn.py	100.00% <100.00%> (ø)

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