code: def _focal_loss(score, label, alpha=0.25, gamma=2.0):
      weight = (score - label).pow(gamma)
      if alpha > 0:
          alpha_t = alpha * label + (1 - alpha) * (1 - label)
          weight *= alpha_t
      loss = F.binary_cross_entropy(
          score, label, weight=weight, reduction="sum"
      )
      return loss
  
imports: import paddle
  import paddle.nn
  import paddle.nn.functional
  from ppdet.core.workspace import register
  from paddle import ParamAttr
  from paddle.nn.initializer import KaimingNormal
  from paddle.nn.initializer import Normal, Constant
  from ..bbox_utils import batch_distance2bbox
  from ..losses import GIoULoss
  from ..initializer import bias_init_with_prob, constant_, normal_
  from ..assigners.utils import generate_anchors_for_grid_cell
  from ppdet.modeling.backbones.cspresnet import ConvBNLayer, RepVggBlock
  from ppdet.modeling.ops import get_static_shape, get_act_fn
  from ppdet.modeling.layers import MultiClassNMS
label_desc: Use image node_id 8 for calling the PPYOLOEHead obj's underlying member method code with example usage: obj._focal_loss(score, label, alpha, gamma) and returns: loss