input: Write a python function to generate combinations from two lists.
  assert generate_combinations([1, 2, 3], [4, 5, 6]) == [[1, 4], [1, 5], [1, 6], [2, 4], [2, 5], [2, 6], [3, 4], [3, 5], [3, 6]]
  assert generate_combinations(['a', 'b'], ['c', 'd']) == [['a', 'c'], ['a', 'd'], ['b', 'c'], ['b', 'd']]
  assert generate_combinations([1], [2, 3]) == [[1, 2], [1, 3]]
output: def generate_combinations(list1, list2):
      return [[x, y] for x in list1 for y in list2]