input: Write a function to perform insertion sort on a given list of integers.
  
  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 = j - 1
    arr[j + 1] = key
  
   return arr
  
  assert insertion_sort([64, 34, 25, 12, 22, 11, 90])==[11, 12, 22, 25, 34, 64, 90]
  assert insertion_sort([3, 1, 4, 1, 5, 9, 2, 6])==[1, 1, 2, 3, 4, 5, 6, 9]
  assert insertion_sort([])==[]
  assert insertion_sort([10, 8, 2, 1, 7, 4])==[1, 2, 4, 7, 8, 10]
  assert insertion_sort([5])==[5]
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 = j - 1
    arr[j + 1] = key
   return arr