input: Write a python function to sum all elements in a 2D array.
  assert sum_2D_array([[1, 2, 3], [4, 5, 6], [7, 8, 9]]) == 45
  assert sum_2D_array([[0, 0, 0], [0, 0, 0], [0, 0, 0]]) == 0
  assert sum_2D_array([[1, -1, 2], [-2, 3, -3], [4, -4, 5]]) == 5
output: def sum_2D_array(arr):
      return sum(sum(row) for row in arr)