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 get_loss(self):
      return self._forward()
  
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 4 for calling the FCOS obj's underlying member method code with example usage: obj.get_loss() and returns: self