Filter fpn backbones for torchvision

[ethanwharris]

What does this PR do?

Fixes #1265

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[codecov]

Codecov Report

Merging #1267 (5f1684d) into master (30436c6) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1267   +/-   ##
=======================================
  Coverage   91.19%   91.19%           
=======================================
  Files         286      286           
  Lines       12791    12793    +2     
=======================================
+ Hits        11665    11667    +2     
  Misses       1126     1126           

Flag	Coverage Δ
unittests	91.19% <100.00%> (+<0.01%)	⬆️

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

Impacted Files	Coverage Δ
flash/core/integrations/icevision/backbones.py	92.85% <100.00%> (+0.54%)	⬆️

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