docstr: Test using custom group_by argument
docstr_tok: ["Test","using","custom","group_by","argument"]
code: def test_custom_group_by(self):
      """Test using custom group_by argument"""
      data = pd.DataFrame(
          data={
              "step": ["a", "b", "c", "d"],
              "best": [0.1, 0.2, 0.3, 0.5],
          }
      )
  
      result = average(data, group_by="step")
  
      assert result.columns.tolist() == ["step", "best_mean"]
  
imports: import numpy
  import pandas
  import pytest
  from sklearn.ensemble import AdaBoostRegressor, BaggingRegressor, ExtraTreesRegressor, GradientBoostingRegressor, RandomForestRegressor
  from orion.analysis.base import average, flatten_numpy, flatten_params, ranking, to_numpy, train_regressor
  from orion.core.io.space_builder import SpaceBuilder
  from orion.core.worker.transformer import build_required_space
label_desc: Use image node_id 6 for calling the TestAverage obj's underlying member method code with example usage: obj.test_custom_group_by() without return types