input: Write a function to sum all elements in a multi-dimensional array.
  assert sum_multi_dim_array([[1, 2, 3], [4, 5, 6], [7, 8, 9]]) == 45
  assert sum_multi_dim_array([[0, 0, 0], [0, 0, 0], [0, 0, 0]]) == 0
  assert sum_multi_dim_array([[1, -1], [1, -1], [1, -1]]) == 0
output: def sum_multi_dim_array(arr):
      return sum(sum(inner_list) for inner_list in arr)