input: Write a function to return a list of tuples, where each tuple contains the index and the square of each number in the given list of integers. If the given 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([-1, -2, -3])==[(0, 1), (1, 4), (2, 9)]
  assert square_index([0])==[(0, 0)]
  assert square_index([])==[]
output: def square_index(nums):
      return [(i, x**2) for i, x in enumerate(nums)]