input: Write a python function to generate all combinations of a given list and return them as a list of tuples.
  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(['a', 'b', 'c'], 2) == [('a', 'b'), ('a', 'c'), ('b', 'c')]
output: def generate_combinations(lst, n):
      from itertools import combinations
      return list(combinations(lst, n))