Fix unittest random failed of test_datasets (#30804)

* fix test_datasets unittest

python/paddle/vision/datasets/flowers.py

@@ -128,6 +128,13 @@ class Flowers(Dataset):
 
         scio = try_import('scipy.io')
 
+        # double check data download
+        self.label_file = _check_exists_and_download(self.label_file, LABEL_URL,
+                                                     LABEL_MD5, 'flowers', True)
+
+        self.setid_file = _check_exists_and_download(self.setid_file, SETID_URL,
+                                                     SETID_MD5, 'flowers', True)
+
         self.labels = scio.loadmat(self.label_file)['labels'][0]
         self.indexes = scio.loadmat(self.setid_file)[self.flag][0]