code: def __len__(self):
      return self.n_groups
  
imports: import warnings
  from collections.abc import Mapping
  import numpy
  import pandas
  import pytorch_lightning
  import torch
  import utilsforecast.processing
  from torch.utils.data import Dataset, DataLoader
  from utilsforecast.compat import DataFrame, pl_Series
label_desc: Use image node_id 3 for calling the TimeSeriesDataset obj's underlying member method code with example usage: obj.__len__() and returns: self