class_docstr: RPN Head
  
  Args:
      anchor_generator (object): `AnchorGenerator` instance
      rpn_target_assign (object): `RPNTargetAssign` instance
      train_proposal (object): `GenerateProposals` instance for training
      test_proposal (object): `GenerateProposals` instance for testing
      num_classes (int): number of classes in rpn output
code: def _get_loss_input(self):
      for attr in [
          "rpn_cls_score",
          "rpn_bbox_pred",
          "anchor",
          "anchor_var",
      ]:
          if not getattr(self, attr, None):
              raise ValueError(
                  "self.{} should not be None,".format(attr),
                  "call RPNHead.get_proposals first",
              )
      return self._transform_input(
          self.rpn_cls_score,
          self.rpn_bbox_pred,
          self.anchor,
          self.anchor_var,
      )
  
imports: from __future__ import absolute_import
  from __future__ import division
  from __future__ import print_function
  from paddle import fluid
  from paddle.fluid.param_attr import ParamAttr
  from paddle.fluid.initializer import Normal
  from paddle.fluid.regularizer import L2Decay
label_desc: Use image node_id 5 for calling the RPNHead obj's underlying member method code with example usage: obj._get_loss_input() and returns: self