input: Write a python function to generate all possible combinations of a list of numbers using itertools.
  assert generate_combinations([1, 2, 3], 3) == [(1, 2, 3)]
  assert generate_combinations([1, 2, 3, 4], 2) == [(1, 2), (1, 3), (1, 4), (2, 3), (2, 4), (3, 4)]
  assert generate_combinations([5, 6], 1) == [(5,), (6,)]
output: def generate_combinations(numbers, length):
      from itertools import combinations
      return list(combinations(numbers, length))