[Tests] reduce the model size in the ddpm fast test (#7797)

* chore: reducing unet size for faster tests

* review suggestions

---------
Co-authored-by: Sayak Paul <spsayakpaul@gmail.com>

tests/pipelines/ddpm/test_ddpm.py

@@ -30,9 +30,10 @@ class DDPMPipelineFastTests(unittest.TestCase):
     def dummy_uncond_unet(self):
         torch.manual_seed(0)
         model = UNet2DModel(
-            block_out_channels=(32, 64),
-            layers_per_block=2,
-            sample_size=32,
+            block_out_channels=(4, 8),
+            layers_per_block=1,
+            norm_num_groups=4,
+            sample_size=8,
             in_channels=3,
             out_channels=3,
             down_block_types=("DownBlock2D", "AttnDownBlock2D"),
@@ -58,10 +59,8 @@ class DDPMPipelineFastTests(unittest.TestCase):
         image_slice = image[0, -3:, -3:, -1]
         image_from_tuple_slice = image_from_tuple[0, -3:, -3:, -1]
 
-        assert image.shape == (1, 32, 32, 3)
-        expected_slice = np.array(
-            [9.956e-01, 5.785e-01, 4.675e-01, 9.930e-01, 0.0, 1.000, 1.199e-03, 2.648e-04, 5.101e-04]
-        )
+        assert image.shape == (1, 8, 8, 3)
+        expected_slice = np.array([0.0, 0.9996672, 0.00329116, 1.0, 0.9995991, 1.0, 0.0060907, 0.00115037, 0.0])
 
         assert np.abs(image_slice.flatten() - expected_slice).max() < 1e-2
         assert np.abs(image_from_tuple_slice.flatten() - expected_slice).max() < 1e-2
@@ -83,7 +82,7 @@ class DDPMPipelineFastTests(unittest.TestCase):
         image_slice = image[0, -3:, -3:, -1]
         image_eps_slice = image_eps[0, -3:, -3:, -1]
 
-        assert image.shape == (1, 32, 32, 3)
+        assert image.shape == (1, 8, 8, 3)
         tolerance = 1e-2 if torch_device != "mps" else 3e-2
         assert np.abs(image_slice.flatten() - image_eps_slice.flatten()).max() < tolerance