yolov5添加损失函数

yolov5添加损失函数的方法是在loss.py文件中新增一个损失函数类，并在该类中定义损失函数的计算方法。可以参考下述代码：

class VFloss(nn.Module):
    def __init__(self, loss_fcn, gamma=1.5, alpha=0.25):
        super(VFloss, self).__init__()
        self.loss_fcn = loss_fcn
        self.gamma = gamma
        self.alpha = alpha
        self.reduction = loss_fcn.reduction
        self.loss_fcn.reduction = 'none'

    def forward(self, pred, true):
        loss = self.loss_fcn(pred, true)
        pred_prob = torch.sigmoid(pred)
        focal_weight = true * (true > 0.0).float() * self.alpha * (pred_prob - true).abs().pow(self.gamma) * (true <= 0.0).float()
        loss *= focal_weight
        if self.reduction == 'mean':
            return loss.mean()
        elif self.reduction == 'sum':
            return loss.sum()
        else:
            return loss