Remove hard coded value from the ASPP layer (#2780)

[AdeelH]

Fixes #2780

[vfdev-5]

Thanks for the PR @AdeelH !
CI failure is unrelated and can be ignored.

[codecov]

Codecov Report

Merging #2793 into master will increase coverage by 0.13%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2793      +/-   ##
==========================================
+ Coverage   73.12%   73.25%   +0.13%     
==========================================
  Files          96       96              
  Lines        8431     8431              
  Branches     1317     1317              
==========================================
+ Hits         6165     6176      +11     
+ Misses       1865     1855      -10     
+ Partials      401      400       -1     

Impacted Files	Coverage Δ
torchvision/models/segmentation/deeplabv3.py	100.00% <ø> (ø)
torchvision/models/detection/rpn.py	93.16% <0.00%> (+0.42%)	⬆️
torchvision/ops/deform_conv.py	70.96% <0.00%> (+3.22%)	⬆️
torchvision/extension.py	68.18% <0.00%> (+18.18%)	⬆️

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