input: Write a function to sort a list of n distinct integers using the insertion sort algorithm. The function should return the sorted list. If the input list is empty, the function should return an empty list.
  assert insertion_sort([12, 11, 13, 5, 6])==[5, 6, 11, 12, 13]
  assert insertion_sort([10, 9, 8, 7, 6, 5])==[5, 6, 7, 8, 9, 10]
  assert insertion_sort([3, 2, 1])==[1, 2, 3]
  assert insertion_sort([1, 2, 3, 4, 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 -= 1
          arr[j + 1] = key
      return arr