input: Write a python function to sort an array using insertion sort algorithm.
  assert insertion_sort([15, -12, 0, 14, 23, -2])==[-12, -2, 0, 14, 15, 23]
  assert insertion_sort([5, 3, 2, 6, 4, 1])==[1, 2, 3, 4, 5, 6]
  assert insertion_sort([1, 2, 3, 4, 5])==[1, 2, 3, 4, 5]
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