input: Write a function to implement insertion sort on a given list of integers. The function should rearrange the elements of the list in non-decreasing order. If the list is empty, it should return an empty 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 = j - 1
    arr[j + 1] = key
  
   return arr
  
  assert insertion_sort([4, 3, 2, 10, 12, 1, 5, 6]) == [1, 2, 3, 4, 5, 6, 10, 12]
  assert insertion_sort([1, 2, 3, 4, 5]) == [1, 2, 3, 4, 5]
  assert insertion_sort([]) == []
  assert insertion_sort([3, 1, 4, 2]) == [1, 2, 3, 4]
  assert insertion_sort([5, 3, 8, 4, 6]) == [3, 4, 5, 6, 8]
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