input: Write a python function to compute the sum of all elements in a 2D array.
  assert sum_2d_array([[1, 2], [3, 4], [5, 6]]) == 21
  assert sum_2d_array([[0, 0], [0, 0]]) == 0
  assert sum_2d_array([[1, -1], [1, -1]]) == 0
  assert sum_2d_array([[10], [20], [30]]) == 60
output: def sum_2d_array(arr):
      return sum(sum(inner) for inner in arr)