code: def test_past_index_generator(self):
      ig = PastCovariatesIndexGenerator(
          self.input_chunk_length, self.output_chunk_length
      )
      self.helper_test_index_types(ig)
      self.helper_test_index_generator_train(ig)
      self.helper_test_index_generator_inference(ig, is_past=True)
  
file_path: darts/darts/tests/dataprocessing/encoders/test_covariate_index_generators.py
imports: import numpy
  import pandas
  import pytest
  from darts import TimeSeries
  from darts.dataprocessing.encoders.encoder_base import CovariatesIndexGenerator, FutureCovariatesIndexGenerator, PastCovariatesIndexGenerator
  from darts.logging import get_logger
  from darts.utils import timeseries_generation
label_desc: Use image node_id 7 for calling the TestCovariatesIndexGenerator obj's underlying member method code with example usage: obj.test_past_index_generator() without return types