input: Write a python function to split a string into chunks of a specified size.
  assert split_str("abcdefghij", 3) == ['abc', 'def', 'ghi', 'j']
  assert split_str("1234567890", 2) == ['12', '34', '56', '78', '90']
  assert split_str("hello world", 5) == ['hello', ' worl', 'd']
output: def split_str(s, size):
      return [s[i:i+size] for i in range(0, len(s), size)]