Initialize bias into YoloHead

[zhiqwang]

Fixing #60, to revert fa3c156

[codecov]

Codecov Report

Merging #67 (7f50575) into master (e5e9012) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #67      +/-   ##
==========================================
+ Coverage   80.08%   80.11%   +0.02%     
==========================================
  Files           8        8              
  Lines         673      674       +1     
==========================================
+ Hits          539      540       +1     
  Misses        134      134              

Flag	Coverage Δ
unittests	80.11% <100.00%> (+0.02%)	⬆️

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Impacted Files	Coverage Δ
test/test_models.py	93.15% <100.00%> (+0.04%)	⬆️

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