Update test for detection model to test input list unmodified

[buoyancy99]

This PR is upon the request for such test in #1084

[fmassa]

LGTM, thanks!

I've kicked the CI again, once tests pass this is good to merge!

[fmassa]

@buoyancy99 test failures are real, can you use instead assertTrue(tensor1.equal(tensor2)?

[codecov-io]

Codecov Report

Merging #1085 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1085      +/-   ##
==========================================
+ Coverage   64.66%   64.67%   +<.01%     
==========================================
  Files          68       68              
  Lines        5406     5407       +1     
  Branches      829      829              
==========================================
+ Hits         3496     3497       +1     
  Misses       1661     1661              
  Partials      249      249

Impacted Files	Coverage Δ
torchvision/models/vgg.py	89.55% <0%> (ø)	⬆️
torchvision/models/resnet.py	88.27% <0%> (ø)	⬆️
torchvision/models/detection/transform.py	78.3% <0%> (+0.2%)	⬆️

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