[Fix] Fix version check in dataset builder. (open-mmlab#474)

* Fix version check in dataset builder.

* Use mmcv version check function.

mmcls/datasets/builder.py

@@ -1,14 +1,13 @@
 # Copyright (c) OpenMMLab. All rights reserved.
 import platform
 import random
-from distutils.version import LooseVersion
 from functools import partial
 
 import numpy as np
 import torch
 from mmcv.parallel import collate
 from mmcv.runner import get_dist_info
-from mmcv.utils import Registry, build_from_cfg
+from mmcv.utils import Registry, build_from_cfg, digit_version
 from torch.utils.data import DataLoader
 
 from .samplers import DistributedSampler
@@ -98,7 +97,7 @@ def build_dataloader(dataset,
         worker_init_fn, num_workers=num_workers, rank=rank,
         seed=seed) if seed is not None else None
 
-    if LooseVersion(torch.__version__) >= LooseVersion('1.7.0'):
+    if digit_version(torch.__version__) >= digit_version('1.8.0'):
         kwargs['persistent_workers'] = persistent_workers
 
     data_loader = DataLoader(