code: def __len__(self):
      return self.length
  
imports: import collections
  import os
  import numpy
  import torch
  from nemo.collections.common.tokenizers import SentencePieceTokenizer, YouTokenToMeTokenizer
  from nemo.collections.nlp.data.language_modeling.megatron.dataset_utils import create_masked_lm_predictions, get_samples_mapping
  from nemo.collections.nlp.data.language_modeling.megatron.gpt_dataset import _build_index_mappings
  from nemo.core import Dataset
label_desc: Use image node_id 2 for calling the MockT5Dataset obj's underlying member method code with example usage: obj.__len__() and returns: self