fix undefined var in test_batch_sampler. test=develop

python/paddle/fluid/tests/unittests/test_batch_sampler.py

@@ -22,6 +22,8 @@ from paddle.io import BatchSampler, Dataset, Sampler, SequenceSampler, \
                         RandomSampler, WeightedRandomSampler
 from paddle.io import DistributedBatchSampler
 
+IMAGE_SIZE = 32
+
 
 class RandomDataset(Dataset):
     def __init__(self, sample_num, class_num):
@@ -31,7 +33,7 @@ class RandomDataset(Dataset):
     def __getitem__(self, idx):
         np.random.seed(idx)
         image = np.random.random([IMAGE_SIZE]).astype('float32')
-        label = np.random.randint(0, CLASS_NUM - 1, (1, )).astype('int64')
+        label = np.random.randint(0, self.class_num - 1, (1, )).astype('int64')
         return image, label
 
     def __len__(self):