input: Write a function to perform a valid insertion sort on a given list of integers. The function should return the sorted list.
  
  assert insertion_sort([4, 3, 2, 10]) == [2, 3, 4, 10]
  assert insertion_sort([12, 11, 13, 5, 6]) == [5, 6, 11, 12, 13]
  assert insertion_sort([]) == []
  assert insertion_sort([1]) == [1]
  assert insertion_sort([5, -1, 3, 2, 1]) == [-1, 1, 2, 3, 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 -= 1
          arr[j + 1] = key
      return arr