class_docstr: An anchor-based head used in `RetinaNet
  <https://arxiv.org/pdf/1708.02002.pdf>`_.
  
  The head contains two subnetworks. The first classifies anchor boxes and
  the second regresses deltas for the anchors.
  
  Example:
      >>> import torch
      >>> self = RetinaHead(11, 7)
      >>> x = torch.rand(1, 7, 32, 32)
      >>> cls_score, bbox_pred = self.forward_single(x)
      >>> # Each anchor predicts a score for each class except background
      >>> cls_per_anchor = cls_score.shape[1] / self.num_anchors
      >>> box_per_anchor = bbox_pred.shape[1] / self.num_anchors
      >>> assert cls_per_anchor == (self.num_classes)
      >>> assert box_per_anchor == 4
docstr: Forward feature of a single scale level.
  
  Args:
      x (Tensor): Features of a single scale level.
  
  Returns:
      tuple:
          cls_score (Tensor): Cls scores for a single scale level
              the channels number is num_anchors * num_classes.
          bbox_pred (Tensor): Box energies / deltas for a single scale
              level, the channels number is num_anchors * 4.
code: def forward_single(self, x):
      """Forward feature of a single scale level.
  
      Args:
          x (Tensor): Features of a single scale level.
  
      Returns:
          tuple:
              cls_score (Tensor): Cls scores for a single scale level
                  the channels number is num_anchors * num_classes.
              bbox_pred (Tensor): Box energies / deltas for a single scale
                  level, the channels number is num_anchors * 4.
      """
      cls_feat = x
      reg_feat = x
      for cls_conv in self.cls_convs:
          cls_feat = cls_conv(cls_feat)
      for reg_conv in self.reg_convs:
          reg_feat = reg_conv(reg_feat)
      cls_score = self.retina_cls(cls_feat)
      bbox_pred = self.retina_reg(reg_feat)
      return cls_score, bbox_pred
  
file_path: mmdetection/mmdet/models/dense_heads/retina_head.py
imports: import torch.nn
  from mmcv.cnn import ConvModule
  from mmdet.registry import MODELS
  from .anchor_head import AnchorHead
label_desc: Use image node_id 3 for calling the RetinaHead obj's underlying member method code with example usage: obj.forward_single(x) and returns: cls_score,  bbox_pred