MobileNetV2 convolution alphagamma crossentropy보완 경량화 모델 Barlow Twins LossFunction Focal loss tensorflow Focal hardexamplemining 클래스 불균형 자동 미분 self-supervised learning MobileNet separable paper review explicit keyword Example Today I Learned friend keyword 하드샘플학습 DeepLearning classimbalance Auto-diff 로스함수 code Gradient Tape modulatingfactor depthwise