class_docstr: Base class for RoI extractor.
  
  Args:
      roi_layer (dict): Specify RoI layer type and arguments.
      out_channels (int): Output channels of RoI layers.
      featmap_strides (int): Strides of input feature maps.
docstr: Build RoI operator to extract feature from each level feature map.
  
  Args:
      layer_cfg (dict): Dictionary to construct and config RoI layer
          operation. Options are modules under ``mmcv/ops`` such as
          ``RoIAlign``.
      featmap_strides (int): The stride of input feature map w.r.t to the
          original image size, which would be used to scale RoI
          coordinate (original image coordinate system) to feature
          coordinate system.
  
  Returns:
      nn.ModuleList: The RoI extractor modules for each level feature
          map.
code: def build_roi_layers(self, layer_cfg, featmap_strides):
      """Build RoI operator to extract feature from each level feature map.
  
      Args:
          layer_cfg (dict): Dictionary to construct and config RoI layer
              operation. Options are modules under ``mmcv/ops`` such as
              ``RoIAlign``.
          featmap_strides (int): The stride of input feature map w.r.t to the
              original image size, which would be used to scale RoI
              coordinate (original image coordinate system) to feature
              coordinate system.
  
      Returns:
          nn.ModuleList: The RoI extractor modules for each level feature
              map.
      """
  
      cfg = layer_cfg.copy()
      layer_type = cfg.pop("type")
      assert hasattr(ops, layer_type)
      layer_cls = getattr(ops, layer_type)
      roi_layers = nn.ModuleList(
          [
              layer_cls(spatial_scale=1 / s, **cfg)
              for s in featmap_strides
          ]
      )
      return roi_layers
  
file_path: insightface/detection/scrfd/mmdet/models/roi_heads/roi_extractors/base_roi_extractor.py
imports: from abc import ABCMeta, abstractmethod
  import torch
  import torch.nn
  from mmcv import ops
label_desc: Use image node_id 4 for calling the BaseRoIExtractor obj's underlying member method code with example usage: obj.build_roi_layers(layer_cfg, featmap_strides) and returns: roi_layers