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_largest_index([5, 10, 20, 6, 8])==2
  assert find_largest_index([15, 25, 5, 10])==1
  assert find_largest_index([2, 3, 3, 2])==1
  assert find_largest_index([7])==0
  assert find_largest_index([])==-1
output: def find_largest_index(lst):
      if not lst:
          return -1
      max_val = max(lst)
      return lst.index(max_val)