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