input: Write a function to add two vectors of the same length element-wise.
  assert vector_add([1, 2, 3], [4, 5, 6]) == [5, 7, 9]
  assert vector_add([0, 0, 0], [1, 2, 3]) == [1, 2, 3]
  assert vector_add([-1, -2, -3], [1, 2, 3]) == [0, 0, 0]
output: def vector_add(vec1, vec2):
      return [x + y for x, y in zip(vec1, vec2)]