input: Write a function to execute a valid insertion sort on a given list of integers. The function should return the sorted list.
  assert insertion_sort([64, 34, 25, 12, 22, 11, 90])==[11, 12, 22, 25, 34, 64, 90]
  assert insertion_sort([5, 4, 3, 2, 1])==[1, 2, 3, 4, 5]
  assert insertion_sort([1, 2, 3, 4, 5])==[1, 2, 3, 4, 5]
  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