class_docstr: FCOS network, see https://arxiv.org/abs/1904.01355
  
  Args:
      backbone (object): backbone instance
      neck (object): 'FPN' instance
      fcos_head (object): 'FCOSHead' instance
      ssod_loss (object): 'SSODFCOSLoss' instance, only used for semi-det(ssod) by DenseTeacher
code: def _forward(self):
      body_feats = self.backbone(self.inputs)
      fpn_feats = self.neck(body_feats)
  
      self.is_teacher = self.inputs.get("is_teacher", False)
      if self.training or self.is_teacher:
          losses = self.fcos_head(fpn_feats, self.inputs)
          return losses
      else:
          fcos_head_outs = self.fcos_head(fpn_feats)
          bbox_pred, bbox_num = self.fcos_head.post_process(
              fcos_head_outs, self.inputs["scale_factor"]
          )
          return {"bbox": bbox_pred, "bbox_num": bbox_num}
  
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 3 for calling the FCOS obj's underlying member method code with example usage: obj._forward() and returns: losses, dict