docstr: ...Test PoissonRegression fit with default parameters
          
docstr_tok: ["...","Test","PoissonRegression","fit","with","default","parameters"]
code: def test_PoissonRegression_fit(self):
      """...Test PoissonRegression fit with default parameters"""
      n_samples = 2000
      n_features = 20
  
      for fit_intercept in [False, True]:
          X, y, weights0, intercept0 = self.get_train_data(
              n_samples=n_samples,
              n_features=n_features,
              fit_intercept=fit_intercept,
          )
  
          learner = PoissonRegression(
              C=1e3,
              verbose=False,
              fit_intercept=fit_intercept,
              solver="bfgs",
          )
          learner.fit(X, y)
          err = np.linalg.norm(learner.weights - weights0) / n_features
          self.assertLess(err, 1e-2)
          if fit_intercept:
              self.assertLess(
                  np.abs(learner.intercept - intercept0), 1e-1
              )
  
imports: import unittest
  from itertools import product
  import numpy
  from tick.base.inference import InferenceTest
  from tick.linear_model import SimuPoisReg, PoissonRegression
  from tick.simulation import weights_sparse_gauss
label_desc: Use image node_id 4 for calling the Test obj's underlying member method code with example usage: obj.test_PoissonRegression_fit() without return types