input: Write a function to find the index of the largest number in a given list of integers. Return the index of the largest number. If the list is empty, return None.
  assert find_max_index([5, 10, 20, 6, 8])==2
  assert find_max_index([3, 4, 5, 1, 2])==2
  assert find_max_index([4, 3, 1, 2])==0
  assert find_max_index([3, 5, 4, 1, 2])==1
  assert find_max_index([])==None
output: def find_max_index(lst):
      if not lst:
          return None
      return lst.index(max(lst))