fix memory consistency decoder test (#5828)

Co-authored-by: Dhruv Nair <dhruv.nair@gmail.com>

tests/models/test_models_vae.py

@@ -804,6 +804,7 @@ class ConsistencyDecoderVAEIntegrationTests(unittest.TestCase):
         gc.collect()
         torch.cuda.empty_cache()
 
+    @torch.no_grad()
     def test_encode_decode(self):
         vae = ConsistencyDecoderVAE.from_pretrained("openai/consistency-decoder")  # TODO - update
         vae.to(torch_device)