input: Write a python function to find the index of the maximum value in a list.
  assert find_max([1, 3, 7, 9, 2]) == 3
  assert find_max([5, 8, 2, 8, 1]) == 1
  assert find_max([10, 20, 30, 20, 10]) == 2
output: def find_max(lst):
      return lst.index(max(lst))