input: 
  def insertion_sort(arr):
      """ Given a list of integers, sort the list using the insertion sort algorithm.
      >>> insertion_sort([64, 34, 25, 12, 22, 11, 90])
      [11, 12, 22, 25, 34, 64, 90]
      >>> insertion_sort([5, 3, 8, 4, 6])
      [3, 4, 5, 6, 8]
      """
      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
  
output:     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