code: def __len__(self):
      return self.samples_mapping.shape[0]
  
file_path: colossalai/examples/tutorial/sequence_parallel/data/datasets/bert_dataset.py
imports: import os
  import time
  import numpy
  import torch
  from torch.utils.data import Dataset
  from colossalai.legacy.context import ParallelMode
  from colossalai.legacy.core import global_context
  from colossalai.logging import get_dist_logger
  from ..tokenizer import get_tokenizer
  from .dataset_utils import create_masked_lm_predictions, create_tokens_and_tokentypes, get_a_and_b_segments, pad_and_convert_to_numpy, truncate_segments
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