input: Write a function that takes a list of integers and returns a list of tuples where each tuple contains the index and the square of each number in the original list. If the input list is empty, return an empty list.
  assert square_index([1, 2, 3, 4, 5])==[(0, 1), (1, 4), (2, 9), (3, 16), (4, 25)]
  assert square_index([0, -1, -2])==[(0, 0), (1, 1), (2, 4)]
  assert square_index([])==[]
output: def square_index(lst):
      return [(i, x**2) for i, x in enumerate(lst)]