Refactor data pipeline

[zhiqwang]

No description provided.

[codecov]

Codecov Report

Merging #88 (31261ce) into master (8597fa8) will increase coverage by 0.57%.
The diff coverage is 99.08%.

@@            Coverage Diff             @@
##           master      #88      +/-   ##
==========================================
+ Coverage   83.31%   83.88%   +0.57%     
==========================================
  Files           9        9              
  Lines         797      813      +16     
==========================================
+ Hits          664      682      +18     
+ Misses        133      131       -2     

Flag	Coverage Δ
unittests	83.88% <99.08%> (+0.57%)	⬆️

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Impacted Files	Coverage Δ
test/test_models.py	94.97% <ø> (+0.49%)	⬆️
test/test_onnx.py	87.95% <93.75%> (+0.17%)	⬆️
test/test_data_pipeline.py	98.27% <100.00%> (+1.21%)	⬆️
test/test_engine.py	99.24% <100.00%> (+<0.01%)	⬆️
test/test_image_utils.py	100.00% <100.00%> (ø)
test/test_utils.py	100.00% <100.00%> (ø)

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