input: Write a function to execute a valid insertion sort on a given list of integers. This function should take a list of integers as input and return the sorted list in non-decreasing order. 
  assert insertion_sort([5, 2, 4, 6, 1, 3])==[1, 2, 3, 4, 5, 6]
  assert insertion_sort([0, 0, 0, 1, 0])==[0, 0, 0, 0, 1]
  assert insertion_sort([])==[]
  assert insertion_sort([1])==[1]
  assert insertion_sort([3, 2])==[2, 3]
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