code: def __len__(self):
      return self.mapping_array.shape[0]
  
imports: import os
  import time
  import numpy
  import torch
  from megatron import print_rank_0
  from megatron.core import mpu, tensor_parallel
  from megatron.data.dataset_utils import create_masked_lm_predictions, pad_and_convert_to_numpy
  from megatron import get_args, get_tokenizer, print_rank_0
label_desc: Use image node_id 2 for calling the BlockSamplesMapping obj's underlying member method code with example usage: obj.__len__() and returns: self