code: def df():
      length = 100
      df = pd.DataFrame(
          {
              TIME_COL: pd.date_range(
                  "2020-01-01", freq="D", periods=length
              ),
              VALUE_COL: np.arange(length),
              PREDICTED_COL: np.arange(length)
              + np.random.randn(length) * length / 10,
          }
      )
      return df
  
imports: import numpy
  import pandas
  import pytest
  from pandas.testing import assert_frame_equal
  from sklearn.metrics import mean_absolute_error
  from sklearn.metrics import mean_squared_error
  from testfixtures import LogCapture
  from greykite.common.constants import LOGGER_NAME
  from greykite.common.constants import PREDICTED_COL
  from greykite.common.constants import PREDICTED_LOWER_COL
  from greykite.common.constants import PREDICTED_UPPER_COL
  from greykite.common.constants import TIME_COL
  from greykite.common.constants import VALUE_COL
  from greykite.common.evaluation import r2_null_model_score
  from greykite.sklearn.estimator.base_forecast_estimator import BaseForecastEstimator
  from greykite.sklearn.uncertainty.uncertainty_methods import UncertaintyMethodEnum
label_desc: Use image node_id 4 for calling a global function with example usage: df() and returns: df