input: Write a function to implement an insertion sort algorithm in Python. The function should take a list of integers as input and return the sorted list in non-decreasing order.
  assert insertion_sort([3, 4, 5, 1, 2])==[1, 2, 3, 4, 5]
  assert insertion_sort([3, 5, 10, 1, 2])==[1, 2, 3, 5, 10]
  assert insertion_sort([4, 3, 1, 2])==[1, 2, 3, 4]
  assert insertion_sort([3, 5, 4, 1, 2])==[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