input: Write a function to check whether a string contains only digits. If the string contains only digits return True else return False.
  assert is_digit_only("0123")==True
  assert is_digit_only("01st")==False
  assert is_digit_only("0")==True
  assert is_digit_only("")==True
  assert is_digit_only("abcd")==False
output: def is_digit_only(s):
      return s.isdigit()