code: def test_flatten_estimator_params_list():
      template = MultistageForecastTemplate()
      x = [
          {"estimator__a": [1], "estimator__b": [2, 3]},
          {"estimator__c": [4, 5]},
      ]
      flattened_params = template._MultistageForecastTemplate__flatten_estimator_params_list(
          estimator_params_list=x
      )
      assert flattened_params == [
          [{"a": 1, "b": 2}, {"c": 4}],
          [{"a": 1, "b": 2}, {"c": 5}],
          [{"a": 1, "b": 3}, {"c": 4}],
          [{"a": 1, "b": 3}, {"c": 5}],
      ]
  
imports: import datetime
  import numpy
  import pytest
  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.testing_utils import generate_df_for_tests
  from greykite.framework.templates.autogen.forecast_config import EvaluationPeriodParam
  from greykite.framework.templates.autogen.forecast_config import ForecastConfig
  from greykite.framework.templates.autogen.forecast_config import MetadataParam
  from greykite.framework.templates.autogen.forecast_config import ModelComponentsParam
  from greykite.framework.templates.forecaster import Forecaster
  from greykite.framework.templates.multistage_forecast_template import MultistageForecastTemplate
  from greykite.framework.templates.multistage_forecast_template_config import SILVERKITE_TWO_STAGE
  from greykite.framework.templates.multistage_forecast_template_config import MultistageForecastTemplateConfig
  from greykite.framework.templates.simple_silverkite_template import SimpleSilverkiteTemplate
  from greykite.sklearn.estimator.simple_silverkite_estimator import SimpleSilverkiteEstimator
  from greykite.sklearn.uncertainty.uncertainty_methods import UncertaintyMethodEnum
label_desc: Use image node_id 13 for calling a global function with example usage: test_flatten_estimator_params_list() without return types