Add a warning if any clip can't be obtained from a video in VideoClips.

[bryant1410]

Fix #2510

[pmeier]

Thanks for the PR @bryant1410! CI failure seems unrelated.

[codecov]

Codecov Report

Merging #2513 (4f9ff20) into master (4521f6d) will decrease coverage by 1.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2513      +/-   ##
==========================================
- Coverage   72.36%   71.29%   -1.07%     
==========================================
  Files          99       94       -5     
  Lines        8817     8296     -521     
  Branches     1385     1398      +13     
==========================================
- Hits         6380     5915     -465     
+ Misses       2005     1945      -60     
- Partials      432      436       +4     

Impacted Files	Coverage Δ
torchvision/datasets/video_utils.py	76.19% <100.00%> (-1.39%)	⬇️
torchvision/datasets/ucf101.py	27.08% <0.00%> (-62.50%)	⬇️
torchvision/ops/feature_pyramid_network.py	75.82% <0.00%> (-18.69%)	⬇️
torchvision/datasets/vision.py	47.36% <0.00%> (-16.43%)	⬇️
torchvision/models/detection/_utils.py	67.34% <0.00%> (-16.10%)	⬇️
torchvision/io/image.py	71.73% <0.00%> (-10.87%)	⬇️
torchvision/io/video.py	69.82% <0.00%> (-10.66%)	⬇️
torchvision/transforms/functional_tensor.py	64.91% <0.00%> (-8.76%)	⬇️
torchvision/datasets/utils.py	56.17% <0.00%> (-8.47%)	⬇️
torchvision/ops/deform_conv.py	66.66% <0.00%> (-5.65%)	⬇️
... and 62 more

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[pmeier]

@bryant1410 Could you add a test that hits the new warning?

[bryant1410]

@bryant1410 Could you add a test that hits the new warning?

Done! (and rebased)

[bjuncek]

Looks good to me.