fix mnist fmnist (#29018)

python/paddle/tests/test_datasets.py

@@ -179,6 +179,13 @@ class TestFASHIONMNISTTrain(unittest.TestCase):
         with self.assertRaises(ValueError):
             mnist = FashionMNIST(mode='train', transform=transform, backend=1)
 
+    def test_dataset_value(self):
+        fmnist = FashionMNIST(mode='train')
+        value = np.mean([np.array(x[0]) for x in fmnist])
+
+        # 72.94035223214286 was getted from competitive products
+        np.testing.assert_allclose(value, 72.94035223214286)
+
 
 class TestFlowersTrain(unittest.TestCase):
     def test_main(self):

python/paddle/vision/datasets/mnist.py

@@ -163,7 +163,7 @@ class MNIST(Dataset):
         image = np.reshape(image, [28, 28])
 
         if self.backend == 'pil':
-            image = Image.fromarray(image, mode='L')
+            image = Image.fromarray(image.astype('uint8'), mode='L')
 
         if self.transform is not None:
             image = self.transform(image)