code: def __len__(self):
      return self.samples_mapping.shape[0]
  
file_path: NeMo/nemo/collections/nlp/data/language_modeling/megatron/bert_dataset.py
imports: import os
  from typing import Any, Optional
  import numpy
  import torch
  from nemo.collections.nlp.data.language_modeling.megatron.dataset_utils import create_masked_lm_predictions, create_tokens_and_tokentypes, get_a_and_b_segments, get_samples_mapping, truncate_segments
  from nemo.collections.nlp.data.language_modeling.megatron.indexed_dataset import MMapIndexedDataset
label_desc: Use image node_id 2 for calling the BertDataset obj's underlying member method code with example usage: obj.__len__() and returns: self