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不使用softmax，开放式分类
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用3个尺度特征来预测，每个尺度的每个像素点预测3个box， $ N\times N \times [3*(4+1+80)] $
4: bbox offset, 1: 是否为object， 80: class
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使用高层特征和底层特征融合 take the feature map from 2 layers previous and upsample it by 2×. We also take a feature map from earlier in the network and merge it with our upsampled features using element-wise addition.
目的: meaningful semantic information + finer-grained information