input: Write a python function to multiply each element in a list by its index.
  assert multiply_by_index([1, 2, 3, 4, 5]) == [0, 2, 6, 12, 20]
  assert multiply_by_index([10, 20, 30, 40, 50]) == [0, 20, 60, 120, 200]
  assert multiply_by_index([0, 1, 2, 3, 4]) == [0, 1, 4, 9, 16]
output: def multiply_by_index(lst):
      return [num * idx for idx, num in enumerate(lst)]