code: def __len__(self):
      return len(self.ds)
  
imports: from typing import Optional, Sequence, Tuple, Union
  import numpy
  from darts import TimeSeries
  from .shifted_dataset import GenericShiftedDataset
  from .training_dataset import DualCovariatesTrainingDataset, FutureCovariatesTrainingDataset, MixedCovariatesTrainingDataset, PastCovariatesTrainingDataset, SplitCovariatesTrainingDataset
  from .utils import CovariateType
label_desc: Use image node_id 2 for calling the PastCovariatesSequentialDataset obj's underlying member method code with example usage: obj.__len__() and returns: len