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 __init__(
      self,
      backbone="ResNet",
      neck="FPN",
      fcos_head="FCOSHead",
      ssod_loss="SSODFCOSLoss",
  ):
      super(FCOS, self).__init__()
      self.backbone = backbone
      self.neck = neck
      self.fcos_head = fcos_head
  
      # for ssod, semi-det
      self.is_teacher = False
      self.ssod_loss = ssod_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 1 to create a new FCOS object from inherited base classes: BaseArch with example: obj = FCOS(backbone, neck, fcos_head, ssod_loss)