【PaddlePaddle】动手实现Focal Loss

最近发现有人问Focal loss怎么实现，我这里分享一下代码，可以自行调节，进行少许的改进，比如给label加上weights，也可以直接使用。

class FocalLoss(nn.Layer):
    def __init__(self, alpha=0.5, gamma=2, weight=None, ignore_index=255):
        super().__init__()
        self.alpha = alpha
        self.gamma = gamma
        # 参数可调
        # self.weight = paddle.to_tensor(np.array([1.063, 4.468, 1.021]))
        self.ignore_index = ignore_index
        self.ce_fn = nn.CrossEntropyLoss(soft_label=False) 
 
    def forward(self, preds, labels):
        logpt = -self.ce_fn(preds, labels)
        pt = paddle.exp(logpt)
        loss = -((1 - pt) ** self.gamma) * self.alpha * logpt
        return loss