input: Write a function to train a Multinomial Naive Bayes classifier using given training data.
  assert train_classifier(X_train, y_train).alpha == 1.0
output: def train_classifier(X_train, y_train):
      from sklearn.naive_bayes import MultinomialNB
      clf = MultinomialNB()
      clf.fit(X_train, y_train)
      return clf