revert default value of allow_empty (#4150)

ppdet/data/source/coco.py

@@ -48,7 +48,7 @@ class COCODataSet(DetDataset):
                  data_fields=['image'],
                  sample_num=-1,
                  load_crowd=False,
-                 allow_empty=True,
+                 allow_empty=False,
                  empty_ratio=1.):
         super(COCODataSet, self).__init__(dataset_dir, image_dir, anno_path,
                                           data_fields, sample_num)

ppdet/data/source/voc.py

@@ -55,7 +55,7 @@ class VOCDataSet(DetDataset):
                  data_fields=['image'],
                  sample_num=-1,
                  label_list=None,
-                 allow_empty=True,
+                 allow_empty=False,
                  empty_ratio=1.):
         super(VOCDataSet, self).__init__(
             dataset_dir=dataset_dir,