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: int: number of feature levels that the generator will be applied
code: def num_levels(self):
      """int: number of feature levels that the generator will be applied"""
      return len(self.base_sizes)
  
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 2 for calling the YOLOAnchorGenerator obj's underlying member method code with example usage: obj.num_levels() and returns: len