code: def test_multiple_columns(self):
      test_data = pd.DataFrame(
          [[1.0, 7.0], [2.0, 8.0], [4.0, 10.0]],
          columns=["series1", "series2"],
          index=pd.to_datetime(
              ["2014-01-01", "2014-01-02", "2014-01-04"]
          ),
      )
      regularized_series = regularization.regularize_series(test_data)
      self.assertIsInstance(regularized_series, pd.DataFrame)
      self.assertAllEqual(regularized_series.shape, [4, 2])
      self.assertAllEqual(
          regularized_series.index,
          pd.to_datetime(
              ["2014-01-01", "2014-01-02", "2014-01-03", "2014-01-04"]
          ),
      )
      self.assertAllEqual(
          regularized_series.columns, ["series1", "series2"]
      )
      self.assertAllEqual(
          regularized_series,
          [[1.0, 7.0], [2.0, 8.0], [np.nan, np.nan], [4.0, 10.0]],
      )
  
file_path: probability/tensorflow_probability/python/sts/regularization_test.py
imports: import datetime
  from absl.testing import parameterized
  import numpy
  import pandas
  from tensorflow_probability.python.internal import test_util
  from tensorflow_probability.python.sts import regularization
label_desc: Use image node_id 8 for calling the RegularizationTest obj's underlying member method code with example usage: obj.test_multiple_columns() without return types