input: Write a function to perform a valid insertion sort on a given list of integers. If the given list is empty then return the same list.
  def insertion_sort(arr):
   for i in range(1, len(arr)):
   key = arr[i]
   j = i -1
   while j >= 0 and key < arr[j] :
      arr[j + 1] = arr[j]
      j -= 1
   arr[j + 1] = key
  
  return arr
  
  assert insertion_sort([5, 3, 8, 6, 4])==[3, 4, 5, 6, 8]
  assert insertion_sort([2, 9, 1, 6])==[1, 2, 6, 9]
  assert insertion_sort([])==[]
  assert insertion_sort([1])==[1]
  assert insertion_sort([3, 2, 1])==[1, 2, 3]
output: def insertion_sort(arr):
   for i in range(1, len(arr)):
    key = arr[i]
    j = i - 1
    while j >= 0 and key < arr[j]:
      arr[j + 1] = arr[j]
      j -= 1
    arr[j + 1] = key
   return arr