input: Write a function to construct a naive bayes classifier using python to classify emails as spam or not spam. The function should return the trained model.
  assert type(naive_bayes_classifier(X_train, y_train))==GaussianNB
  assert list(naive_bayes_classifier(X_train, y_train).predict([[0, 1, 1], [1, 0, 0]]))==[0, 1]
output: def naive_bayes_classifier(X_train, y_train):
      from sklearn.naive_bayes import GaussianNB
      model = GaussianNB()
      model.fit(X_train, y_train)
      return model