input: Write a python function to generate all combinations of the characters 'a' and 'b' of a given length using itertools.product.
  assert generate_combinations(2) == ['aa', 'ab', 'ba', 'bb']
  assert generate_combinations(3) == ['aaa', 'aab', 'aba', 'abb', 'baa', 'bab', 'bba', 'bbb']
  assert generate_combinations(1) == ['a', 'b']
output: def generate_combinations(length):
      from itertools import product
      return [''.join(comb) for comb in product('ab', repeat=length)]