input: Write a function to sort a list of n distinct integers using the insertion sort algorithm. The function should return the sorted list.
  assert insertion_sort([12, 11, 13, 5, 6])==[5, 6, 11, 12, 13]
  assert insertion_sort([3, 7, 4, 9, 5, 2, 6, 1])==[1, 2, 3, 4, 5, 6, 7, 9]
  assert insertion_sort([64, 25, 12, 22, 11])==[11, 12, 22, 25, 64]
  assert insertion_sort([])==[]
  assert insertion_sort([7])==[7]
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