class_docstr: This class represents masks in the form of bitmaps.
  
  Args:
      masks (ndarray): ndarray of masks in shape (N, H, W), where N is
          the number of objects.
      height (int): height of masks
      width (int): width of masks
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docstr: Index the BitmapMask.
  
  Args:
      index (int | ndarray): Indices in the format of integer or ndarray.
  
  Returns:
      :obj:`BitmapMasks`: Indexed bitmap masks.
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code: def __getitem__(self, index):
      """Index the BitmapMask.
  
      Args:
          index (int | ndarray): Indices in the format of integer or ndarray.
  
      Returns:
          :obj:`BitmapMasks`: Indexed bitmap masks.
      """
      masks = self.masks[index].reshape(-1, self.height, self.width)
      return BitmapMasks(masks, self.height, self.width)
  
imports: from abc import ABCMeta, abstractmethod
  import cv2
  import mmcv
  import numpy
  import pycocotools.mask
  import torch
  from mmcv.ops.roi_align import roi_align
label_desc: Use image node_id 2 for calling the BitmapMasks obj's underlying member method code with example usage: obj.__getitem__(index) and returns: BitmapMasks