class_docstr: SSDHead
  
  Args:
      num_classes (int): Number of classes
      in_channels (list): Number of channels per input feature
      anchor_generator (dict): Configuration of 'AnchorGeneratorSSD' instance
      kernel_size (int): Conv kernel size
      padding (int): Conv padding
      use_sepconv (bool): Use SepConvLayer if true
      conv_decay (float): Conv regularization coeff
      loss (object): 'SSDLoss' instance
      use_extra_head (bool): If use ResNet34 as baskbone, you should set `use_extra_head`=True
code: def get_loss(self, boxes, scores, gt_bbox, gt_class, prior_boxes):
      return self.loss(boxes, scores, gt_bbox, gt_class, prior_boxes)
  
imports: import paddle
  import paddle.nn
  import paddle.nn.functional
  from ppdet.core.workspace import register
  from paddle.regularizer import L2Decay
  from paddle import ParamAttr
  from ..layers import AnchorGeneratorSSD
  from ..cls_utils import _get_class_default_kwargs
label_desc: Use image node_id 4 for calling the SSDHead obj's underlying member method code with example usage: obj.get_loss(boxes, scores, gt_bbox, gt_class, prior_boxes) and returns: self