input: Write a function to find the index of the largest number in a given list of integers. If the list is empty, return -1.
  assert find_max_index([5, 10, 20, 6, 8]) == 2
  assert find_max_index([1, 2, 3, 4, 5]) == 4
  assert find_max_index([5, 5, 5, 5, 5]) == 0
  assert find_max_index([-1, -2, -3, -4, -5]) == 0
  assert find_max_index([]) == -1
output: def find_max_index(lst):
      if not lst:
          return -1
      max_val = max(lst)
      return lst.index(max_val)