class_docstr: FCOS ARSL network, see https://arxiv.org/abs/
  
  Args:
      backbone (object): backbone instance
      neck (object): 'FPN' instance
      fcos_head (object): 'FCOSHead_ARSL' instance
      fcos_cr_loss (object): 'FCOSLossCR' instance, only used for semi-det(ssod) by ARSL
code: def get_loss(self):
      loss = {}
      tag_labels, tag_bboxes, tag_centerness = [], [], []
      for i in range(len(self.fcos_head.fpn_stride)):
          # labels, reg_target, centerness
          k_lbl = "labels{}".format(i)
          if k_lbl in self.inputs:
              tag_labels.append(self.inputs[k_lbl])
          k_box = "reg_target{}".format(i)
          if k_box in self.inputs:
              tag_bboxes.append(self.inputs[k_box])
          k_ctn = "centerness{}".format(i)
          if k_ctn in self.inputs:
              tag_centerness.append(self.inputs[k_ctn])
      fcos_head_outs = self.model_forward()
      loss_fcos = self.fcos_head.get_loss(
          fcos_head_outs, tag_labels, tag_bboxes, tag_centerness
      )
      loss.update(loss_fcos)
      return loss
  
imports: from __future__ import absolute_import
  from __future__ import division
  from __future__ import print_function
  import paddle
  from ppdet.core.workspace import register, create
  from .meta_arch import BaseArch
label_desc: Use image node_id 5 for calling the ARSL_FCOS obj's underlying member method code with example usage: obj.get_loss() and returns: loss