class_docstr: Anchor generator for YOLO.
  
  Args:
      strides (list[int] | list[tuple[int, int]]): Strides of anchors
          in multiple feature levels.
      base_sizes (list[list[tuple[int, int]]]): The basic sizes
          of anchors in multiple levels.
docstr: Generate base anchors.
  
  Returns:
      list(torch.Tensor): Base anchors of a feature grid in multiple                 feature levels.
code: def gen_base_anchors(self):
      """Generate base anchors.
  
          Returns:
              list(torch.Tensor): Base anchors of a feature grid in multiple \
                  feature levels.
          """
      multi_level_base_anchors = []
      for i, base_sizes_per_level in enumerate(self.base_sizes):
          center = None
          if self.centers is not None:
              center = self.centers[i]
          multi_level_base_anchors.append(
              self.gen_single_level_base_anchors(
                  base_sizes_per_level, center
              )
          )
      return multi_level_base_anchors
  
imports: import mmcv
  import numpy
  import torch
  from torch.nn.modules.utils import _pair
  from .builder import ANCHOR_GENERATORS
label_desc: Use image node_id 3 for calling the YOLOAnchorGenerator obj's underlying member method code with example usage: obj.gen_base_anchors() and returns: multi_level_base_anchors