code: def test_multi_timeseries_variable_ts_length_one_ts_too_small(self):
      data = [
          constant_timeseries(value=123, length=21),
          constant_timeseries(value=123, length=100),
          constant_timeseries(value=123, length=1000),
      ]
      with pytest.raises(AttributeError) as err:
          train_set, test_set = train_test_split(
              data,
              axis=1,
              test_size=2,
              input_size=1,
              horizon=18,
              vertical_split_type=MODEL_AWARE,
          )
      assert (
          str(err.value)
          == "Not enough data to create training and test sets"
      )
  
imports: import pytest
  from darts.utils.model_selection import MODEL_AWARE, SIMPLE, train_test_split
  from darts.utils.timeseries_generation import constant_timeseries
label_desc: Use image node_id 20 for calling the TestClassTrainTestSplit obj's underlying member method code with example usage: obj.test_multi_timeseries_variable_ts_length_one_ts_too_small() without return types