code: def test_flat_trend(self, backend):
      model = Prophet(stan_backend=backend)
      t = np.arange(11)
      m = 0.5
      y = model.flat_trend(t, m)
      y_true = np.array([0.5] * 11)
      assert (y - y_true).sum() == 0.0
      t = t[8:]
      y_true = y_true[8:]
      y = model.flat_trend(t, m)
      assert (y - y_true).sum() == 0.0
  
imports: import numpy
  import pandas
  import pytest
  from prophet import Prophet
  from prophet.utilities import warm_start_params
label_desc: Use image node_id 7 for calling the TestProphetTrendComponent obj's underlying member method code with example usage: obj.test_flat_trend(backend) without return types