code: def __len__(self):
      return self.total_samples
  
imports: import random
  import torch
  import numpy
  from torch.utils.data import Dataset
  from megatron import get_args
  from megatron.core import mpu
label_desc: Use image node_id 2 for calling the MegatronPretrainingSampler obj's underlying member method code with example usage: obj.__len__() and returns: self