input: Write a function to find the index of the largest number in a given list of integers. Return the index as an integer. If the list is empty, return -1.
  assert list_max_index([5, 10, 20, 6, 8])==2
  assert list_max_index([3, 7, 12, 5, 9])==2
  assert list_max_index([4])==0
  assert list_max_index([])==-1
output: def list_max_index(lst):
      if not lst:
          return -1
      return lst.index(max(lst))