def binary_cross_entropy(pred,
                         label,
                         weight=None,
                         reduction='mean',
                         avg_factor=None,
                         class_weight=None,
                         ignore_index=-100,
                         avg_non_ignore=False):
    """Calculate the binary CrossEntropy loss.

    Args:
        pred (torch.Tensor): The prediction with shape (N, 1) or (N, ).
            When the shape of pred is (N, 1), label will be expanded to
            one-hot format, and when the shape of pred is (N, ), label
            will not be expanded to one-hot format.
        label (torch.Tensor): The learning label of the prediction,
            with shape (N, ).
        weight (torch.Tensor, optional): Sample-wise loss weight.
        reduction (str, optional): The method used to reduce the loss.
            Options are "none", "mean" and "sum".
        avg_factor (int, optional): Average factor that is used to average
            the loss. Defaults to None.
        class_weight (list[float], optional): The weight for each class.
        ignore_index (int | None): The label index to be ignored.
            If None, it will be set to default value. Default: -100.
        avg_non_ignore (bool): The flag decides to whether the loss is
            only averaged over non-ignored targets. Default: False.

    Returns:
        torch.Tensor: The calculated loss.
    """
    # The default value of ignore_index is the same as F.cross_entropy
    ignore_index = -100 if ignore_index is None else ignore_index

    if pred.dim() != label.dim():
        label, weight, valid_mask = _expand_onehot_labels(
            label, weight, pred.size(-1), ignore_index)
    else:
        # should mask out the ignored elements
        valid_mask = ((label >= 0) & (label != ignore_index)).float()
        if weight is not None:
            # The inplace writing method will have a mismatched broadcast
            # shape error if the weight and valid_mask dimensions
            # are inconsistent such as (B,N,1) and (B,N,C).
            weight = weight * valid_mask
        else:
            weight = valid_mask

    # average loss over non-ignored elements
    if (avg_factor is None) and avg_non_ignore and reduction == 'mean':
        avg_factor = valid_mask.sum().item()

    # weighted element-wise losses
    weight = weight.float()
    loss = F.binary_cross_entropy_with_logits(
        pred, label.float(), pos_weight=class_weight, reduction='none')
    # do the reduction for the weighted loss
    loss = weight_reduce_loss(
        loss, weight, reduction=reduction, avg_factor=avg_factor)

    return loss
Calculate the binary CrossEntropy loss.

Args:
pred (torch.Tensor): The prediction with shape (N, 1) or (N, ).
When the shape of pred is (N, 1), label will be expanded to
one-hot format, and when the shape of pred is (N, ), label
will not be expanded to one-hot format.
label (torch.Tensor): The learning label of the prediction,
with shape (N, ).
weight (torch.Tensor, optional): Sample-wise loss weight.
reduction (str, optional): The method used to reduce the loss.
Options are "none", "mean" and "sum".
avg_factor (int, optional): Average factor that is used to average
the loss. Defaults to None.
class_weight (list[float], optional): The weight for each class.
ignore_index (int | None): The label index to be ignored.
If None, it will be set to default value. Default: -100.
avg_non_ignore (bool): The flag decides to whether the loss is
only averaged over non-ignored targets. Default: False.

Returns:
torch.Tensor: The calculated loss.
third_parts/mmdet/models/losses/cross_entropy_loss.py