Update faster_rcnn.py

[dldldlfma]

Add 'import torch' in example

[codecov-io]

Codecov Report

Merging #979 into master will increase coverage by 0.05%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #979      +/-   ##
==========================================
+ Coverage    61.1%   61.16%   +0.05%     
==========================================
  Files          65       65              
  Lines        5091     5091              
  Branches      764      764              
==========================================
+ Hits         3111     3114       +3     
+ Misses       1769     1767       -2     
+ Partials      211      210       -1

Impacted Files	Coverage Δ
torchvision/models/detection/faster_rcnn.py	74.39% <ø> (ø)	⬆️
torchvision/transforms/transforms.py	82.54% <0%> (+0.64%)	⬆️

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[fmassa]

Thanks!