input: Write a python function to generate every possible combination of given characters using the itertools.product method.
  assert set(generate_combinations(["a", "b", "c"])) == set(['a', 'b', 'c'])
  assert set(generate_combinations(["x", "y"])) == set(['x', 'y'])
  assert set(generate_combinations(["p", "q", "r", "s"])) == set(['p', 'q', 'r', 's'])
output: from itertools import product
  
  def generate_combinations(chars):
      return [''.join(p) for p in product(chars, repeat=1)]