input: Write a python function to train and evaluate a Multinomial Naive Bayes model using scikit-learn.
  assert train_evaluate_model(X_train, y_train, X_test, y_test) == accuracy_value
  assert train_evaluate_model(X_train2, y_train2, X_test2, y_test2) == accuracy_value2
output: def train_evaluate_model(X_train, y_train, X_test, y_test):
      from sklearn.naive_bayes import MultinomialNB
      from sklearn.metrics import accuracy_score
  
      model = MultinomialNB()
      model.fit(X_train, y_train)
      y_pred = model.predict(X_test)
      accuracy = accuracy_score(y_test, y_pred)
      
      return accuracy