input: Write a function to sort a given list of integers using the insertion sort algorithm.
  
  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([3, 1, 2, 5, 4]) == [1, 2, 3, 4, 5]
  assert insertion_sort([5, 3, 1, 2, 4]) == [1, 2, 3, 4, 5]
  assert insertion_sort([1, 4, 2, 3, 5]) == [1, 2, 3, 4, 5]
  assert insertion_sort([2, 1, 4, 3, 5]) == [1, 2, 3, 4, 5]
  assert insertion_sort([]) == []
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