input: Write a python function to convert an integer to a string and a string to an integer.
  assert NumStringConverter().int_to_str(123) == '123'
  assert NumStringConverter().int_to_str(0) == '0'
  assert NumStringConverter().str_to_int('456') == 456
  assert NumStringConverter().str_to_int('0') == 0
output: class NumStringConverter:
      def int_to_str(self, num: int) -> str:
          return str(num)
  
      def str_to_int(self, s: str) -> int:
          return int(s)