code: def __len__(self):
      if self.max_num_samples is None:
          return len(self.indexed_dataset)
      else:
          return len(self.samples_mapping)
  
imports: import re
  from typing import List, Mapping, Optional
  import datasets
  import numpy
  import torch
  from datasets import load_dataset
  from nemo.collections.common.tokenizers.tokenizer_spec import TokenizerSpec
  from nemo.collections.nlp.data.language_modeling.megatron.dataset_utils import get_samples_mapping
  from nemo.collections.nlp.data.language_modeling.text_memmap_dataset import JSONLMemMapDataset
  from nemo.core.classes import Dataset
  from nemo.utils import logging
label_desc: Use image node_id 4 for calling the GPTSFTDataset obj's underlying member method code with example usage: obj.__len__() and returns: len, len