Fix deprecation warning in nonzero

[fmassa]

Fixes #2704 and #2154 and subsumes #2314

Instead of passing as_tuple keyword to nonzero, we instead replace nonzero by where, now that it works with just the condition argument.

This should hopefully work with torchscript and ONNX.

[codecov]

Codecov Report

Merging #2705 into master will decrease coverage by 0.13%.
The diff coverage is 87.50%.

@@            Coverage Diff             @@
##           master    #2705      +/-   ##
==========================================
- Coverage   72.82%   72.68%   -0.14%     
==========================================
  Files          95       95              
  Lines        8212     8212              
  Branches     1283     1283              
==========================================
- Hits         5980     5969      -11     
- Misses       1838     1848      +10     
- Partials      394      395       +1     

Impacted Files	Coverage Δ
torchvision/models/detection/_utils.py	85.03% <50.00%> (ø)
torchvision/models/detection/generalized_rcnn.py	88.52% <100.00%> (ø)
torchvision/models/detection/roi_heads.py	82.34% <100.00%> (ø)
torchvision/models/detection/rpn.py	92.70% <100.00%> (-0.43%)	⬇️
torchvision/ops/boxes.py	100.00% <100.00%> (ø)
torchvision/ops/poolers.py	97.05% <100.00%> (ø)
torchvision/extension.py	48.64% <0.00%> (-21.63%)	⬇️
torchvision/ops/deform_conv.py	66.66% <0.00%> (-3.34%)	⬇️

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