Fix loss computation

[zhiqwang]

Closes #16

[codecov]

Codecov Report

Merging #25 (860e4d5) into master (a432f74) will increase coverage by 2.71%.
The diff coverage is 43.52%.

@@            Coverage Diff             @@
##           master      #25      +/-   ##
==========================================
+ Coverage   34.30%   37.02%   +2.71%     
==========================================
  Files          31       33       +2     
  Lines        2670     2847     +177     
==========================================
+ Hits          916     1054     +138     
- Misses       1754     1793      +39     

Flag	Coverage Δ
unittests	37.02% <43.52%> (+2.71%)	⬆️

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Impacted Files	Coverage Δ
datasets/coco.py	0.00% <0.00%> (ø)
datasets/transforms.py	0.00% <0.00%> (ø)
detect.py	0.00% <0.00%> (ø)
engine.py	0.00% <0.00%> (ø)
test/dataset_utils.py	0.00% <0.00%> (ø)
utils/builtin_meta.py	0.00% <0.00%> (ø)
utils/image_utils.py	0.00% <0.00%> (ø)
utils/misc.py	0.00% <0.00%> (ø)
utils/yolo2coco.py	0.00% <0.00%> (ø)
models/transform.py	86.84% <86.84%> (ø)
... and 15 more

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