Make CocoDataset ids deterministically ordered

[fmassa]

The keys of a dict do not have a guaranteed order.
Sorting them to ensure that the order is consistent.

[codecov-io]

Codecov Report

Merging #868 into master will decrease coverage by 0.03%.
The diff coverage is 0%.

@@            Coverage Diff            @@
##           master    #868      +/-   ##
=========================================
- Coverage   54.43%   54.4%   -0.04%     
=========================================
  Files          36      36              
  Lines        3323    3323              
  Branches      546     546              
=========================================
- Hits         1809    1808       -1     
- Misses       1379    1380       +1     
  Partials      135     135

Impacted Files	Coverage Δ
torchvision/datasets/coco.py	24.48% <0%> (ø)	⬆️
torchvision/transforms/functional.py	69.2% <0%> (-1.27%)	⬇️
torchvision/transforms/transforms.py	82.68% <0%> (+0.68%)	⬆️

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