code: def fs():
      """feature transformation pipeline for test cases"""
      return PandasFeatureUnion(
          [
              (
                  "date",
                  Pipeline(
                      [
                          (
                              "select_date",
                              ColumnSelector([TIME_COL]),
                          )  # leaves time column unmodified
                      ]
                  ),
              ),
              (
                  "response",
                  Pipeline(
                      [  # applies outlier and null transformation to value column
                          ("select_val", ColumnSelector([VALUE_COL])),
                          ("outlier", ZscoreOutlierTransformer()),
                          ("null", NullTransformer()),
                      ]
                  ),
              ),
          ]
      )
  
imports: import numpy
  import pandas
  import pytest
  from sklearn.metrics import mean_squared_error
  from sklearn.pipeline import Pipeline
  from greykite.common.constants import ACTUAL_COL
  from greykite.common.constants import TIME_COL
  from greykite.common.constants import VALUE_COL
  from greykite.common.python_utils import assert_equal
  from greykite.sklearn.estimator.null_model import DummyEstimator
  from greykite.sklearn.estimator.simple_silverkite_estimator import SimpleSilverkiteEstimator
  from greykite.sklearn.transform.column_selector import ColumnSelector
  from greykite.sklearn.transform.difference_based_outlier_transformer import DifferenceBasedOutlierTransformer
  from greykite.sklearn.transform.null_transformer import NullTransformer
  from greykite.sklearn.transform.pandas_feature_union import PandasFeatureUnion
  from greykite.sklearn.transform.zscore_outlier_transformer import ZscoreOutlierTransformer
label_desc: Use image node_id 2 for calling a global function with example usage: fs() and returns: PandasFeatureUnion