code: def __len__(self):
      return (
          len(self.data_source) + self.batch_size - 1
      ) // self.batch_size
  
imports: import json
  import pickle
  import random
  import dgl
  import numpy
  import torch
label_desc: Use image node_id 3 for calling the BucketSampler obj's underlying member method code with example usage: obj.__len__() and returns: unknown