input: Write a function to sum the digits of two numbers.
  assert sum_digits(123, 456) == 21
  assert sum_digits(111, 222) == 9
  assert sum_digits(0, 0) == 0
output: def sum_digits(num1, num2):
      def digit_sum(n):
          return sum(int(digit) for digit in str(n))
      return digit_sum(num1) + digit_sum(num2)