code: def test_constant_regressor(self, daily_univariate_ts, backend):
      df = daily_univariate_ts.copy()
      df["constant_feature"] = 0
      m = Prophet(stan_backend=backend)
      m.add_regressor("constant_feature")
      m.fit(df)
      assert m.extra_regressors["constant_feature"]["std"] == 1
  
imports: import numpy
  import pandas
  import pytest
  from prophet import Prophet
  from prophet.utilities import warm_start_params
label_desc: Use image node_id 2 for calling the TestProphetRegressors obj's underlying member method code with example usage: obj.test_constant_regressor(daily_univariate_ts, backend) without return types