input: Write a python function to find the smallest item in a 2D array.
  assert find_smallest([[1, 2, 3], [4, 5, 6], [7, 8, 9]]) == 1
  assert find_smallest([[10, 20, 30], [5, 15, 25], [50, 60, 70]]) == 5
  assert find_smallest([[3, 8, 2], [9, 7, 4], [6, 1, 5]]) == 1
output: def find_smallest(arr):
      return min(min(row) for row in arr)