input: Write a function to calculate the sum of the digits in a given number. If the given number is a negative, ignore the negative sign. If the given number is 0, return 0.
  assert sum_digits(12345)==15
  assert sum_digits(1111)==4
  assert sum_digits(-12345)==15
  assert sum_digits(0)==0
output: def sum_digits(num):
      num = abs(num)
      return sum(int(digit) for digit in str(num))