input: Write a python function to create and train a decision tree classifier using scikit-learn.
  assert train_decision_tree([[0, 0], [1, 1]], [0, 1]).predict([[2., 2.]])[0] == 1
  assert train_decision_tree([[0, 0], [1, 1], [1, 0], [0, 1]], [0, 1, 0, 1]).predict([[0., 1.]])[0] == 1
  assert train_decision_tree([[2, 3], [1, 5], [3, 1], [6, 0]], [0, 1, 0, 1]).predict([[4, 2]])[0] == 1
output: def train_decision_tree(X, y):
      from sklearn.tree import DecisionTreeClassifier
      clf = DecisionTreeClassifier()
      clf.fit(X, y)
      return clf