input: Write a Python function to create an array of zeros with a given length and then assign each element its index value.
  ```python
  assert np.array_equal(create_index_array(4), np.array([0, 1, 2, 3]))
  assert np.array_equal(create_index_array(6), np.array([0, 1, 2, 3, 4, 5]))
  assert np.array_equal(create_index_array(0), np.array([]))
output: def create_index_array(length):
      return np.arange(length)