input: Write a function to check whether a given string contains only digits. If the string contains only digits, return True, otherwise return False.
  assert is_digit_only("0123")==True
  assert is_digit_only("12a3")==False
  assert is_digit_only("9876543210")==True
  assert is_digit_only("123 456")==False
  assert is_digit_only("")==True
output: def is_digit_only(s):
      return s.isdigit()