Remove cpp extensions in favor of torch ops

[fmassa]

Fixes #1322

Now we only use torch operators, which makes it possible for them to be available in torchscript.

I also handle the case of TorchHub support, because now instead of lazily importing the extensions, we unconditionally import the extensions at torchvision import time, but we don't fail hard if it's not available

[codecov-io]

Codecov Report

Merging #1348 into master will decrease coverage by 0.02%.
The diff coverage is 72.97%.

@@            Coverage Diff             @@
##           master    #1348      +/-   ##
==========================================
- Coverage    65.5%   65.48%   -0.03%     
==========================================
  Files          75       75              
  Lines        5819     5827       +8     
  Branches      892      892              
==========================================
+ Hits         3812     3816       +4     
- Misses       1737     1743       +6     
+ Partials      270      268       -2

Impacted Files	Coverage Δ
torchvision/ops/boxes.py	94.44% <ø> (-0.3%)	⬇️
torchvision/__init__.py	76.19% <100%> (+1.19%)	⬆️
torchvision/ops/roi_pool.py	72% <100%> (-2.08%)	⬇️
torchvision/ops/roi_align.py	69.23% <100%> (-2.2%)	⬇️
torchvision/ops/__init__.py	100% <100%> (ø)	⬆️
torchvision/ops/_register_onnx_ops.py	100% <100%> (ø)
torchvision/ops/_utils.py	33.33% <33.33%> (+6.06%)	⬆️
torchvision/extension.py	48.48% <71.42%> (+7.57%)	⬆️
torchvision/models/detection/roi_heads.py	55.77% <0%> (-0.16%)	⬇️
torchvision/models/detection/rpn.py	79.32% <0%> (+0.09%)	⬆️
... and 1 more

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