code: def test_unweighted_ignore_class_255(self):
      y_pred = [0, 1, 1, 1]
      y_true = [0, 1, 2, 255]
  
      m_obj = metrics.MeanIoU(num_classes=3, ignore_class=255)
  
      result = m_obj(y_true, y_pred)
  
      # cm = [[1, 0, 0],
      #       [0, 1, 0],
      #       [0, 1, 0]]
      # sum_row = [1, 1, 1], sum_col = [1, 2, 0], true_positives = [1, 1, 0]
      # iou = true_positives / (sum_row + sum_col - true_positives))
      expected_result = (
          1 / (1 + 1 - 1) + 1 / (2 + 1 - 1) + 0 / (0 + 1 - 0)
      ) / 3
      self.assertAllClose(result, expected_result, atol=1e-3)
  
imports: import numpy
  import pytest
  from keras import layers
  from keras import models
  from keras import testing
  from keras.metrics import iou_metrics
label_desc: Use image node_id 3 for calling the MeanIoUTest obj's underlying member method code with example usage: obj.test_unweighted_ignore_class_255() without return types