code: def test_zero_feature():
      zf = ZeroFeature()
      feature = zf(
          pd.date_range("2020-01-01 21:00:00", periods=10, freq="H")
      )
  
      assert isinstance(feature, np.ndarray)
      assert feature.shape == (10,)
      assert feature.dtype == np.float
  
file_path: gluonts/test/mx/model/deepstate/test_issm.py
imports: import mxnet
  import numpy
  import pandas
  from gluonts.mx.model.deepstate.issm import CompositeISSM, LevelISSM, LevelTrendISSM, SeasonalityISSM, ZeroFeature
  from gluonts.time_feature import day_of_week_index, month_of_year_index
label_desc: Use image node_id 1 for calling a global function with example usage: test_zero_feature() without return types