input: Write a function that takes a list of integers (nums) and returns a list of tuples where each tuple contains the index of the number and the square of the number from the original list. Use list comprehension and the enumerate function to achieve this. 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([])==[]
output: def square_index(nums):
      return [(i, x**2) for i, x in enumerate(nums)]