bug fix for init sampler during testing

mmdet/datasets/loader/build_loader.py

@@ -24,13 +24,14 @@ def build_dataloader(dataset,
                                           rank)
         batch_size = imgs_per_gpu
         num_workers = workers_per_gpu
+    else:
+        if not kwargs.get('shuffle', True):
+            sampler = None
         else:
             sampler = GroupSampler(dataset, imgs_per_gpu)
         batch_size = num_gpus * imgs_per_gpu
         num_workers = num_gpus * workers_per_gpu
 
-    if not kwargs.get('shuffle', True):
-        sampler = None
 
     data_loader = DataLoader(
         dataset,