input: Write a python function to compute the pairwise sum of two lists.
  assert pairwise_sum([1, 2, 3, 4], [4, 5, 6, 7]) == [5, 7, 9, 11]
  assert pairwise_sum([0, -1, -2], [1, 1, 1]) == [1, 0, -1]
  assert pairwise_sum([10, 20], [30, 40]) == [40, 60]
output: def pairwise_sum(list1, list2):
      return [x + y for x, y in zip(list1, list2)]