[Fix] Fix a bug about multi-class in VideoDataset (open-mmlab#723)

* Fix 722

* add unittest and update changelog

docs/changelog.md

@@ -16,6 +16,8 @@
 
 **Bug and Typo Fixes**
 
+- Fix a bug about multi-class in VideoDataset ([#723](https://github.com/open-mmlab/mmaction2/pull/678))
+
 **ModelZoo**
 
 - Add LFB for AVA2.1 ([#553](https://github.com/open-mmlab/mmaction2/pull/553))

mmaction/datasets/video_dataset.py

@@ -1,7 +1,5 @@
 import os.path as osp
 
-import torch
-
 from .base import BaseDataset
 from .registry import DATASETS
 
@@ -53,15 +51,10 @@ def load_annotations(self):
                     assert self.num_classes is not None
                     filename, label = line_split[0], line_split[1:]
                     label = list(map(int, label))
-                    onehot = torch.zeros(self.num_classes)
-                    onehot[label] = 1.0
                 else:
                     filename, label = line_split
                     label = int(label)
                 if self.data_prefix is not None:
                     filename = osp.join(self.data_prefix, filename)
-                video_infos.append(
-                    dict(
-                        filename=filename,
-                        label=onehot if self.multi_class else label))
+                video_infos.append(dict(filename=filename, label=label))
         return video_infos

tests/data/annotations/video_test_list_multi_label.txt

@@ -0,0 +1,2 @@
+test.mp4 0 3
+test.mp4 0 2 4

tests/test_data/test_datasets/base.py

@@ -41,6 +41,8 @@ def setup_class(cls):
                                               'rawvideo_test_anno.txt')
         cls.video_ann_file = osp.join(cls.ann_file_prefix,
                                       'video_test_list.txt')
+        cls.video_ann_file_multi_label = osp.join(
+            cls.ann_file_prefix, 'video_test_list_multi_label.txt')
 
         # pipeline configuration
         cls.action_pipeline = []

tests/test_data/test_datasets/test_video_dataset.py

@@ -28,6 +28,24 @@ def test_video_dataset(self):
         assert video_infos == [dict(filename=video_filename, label=0)] * 2
         assert video_dataset.start_index == 0
 
+    def test_video_dataset_multi_label(self):
+        video_dataset = VideoDataset(
+            self.video_ann_file_multi_label,
+            self.video_pipeline,
+            data_prefix=self.data_prefix,
+            multi_class=True,
+            num_classes=100)
+        video_infos = video_dataset.video_infos
+        video_filename = osp.join(self.data_prefix, 'test.mp4')
+        label0 = [0, 3]
+        label1 = [0, 2, 4]
+        labels = [label0, label1]
+        for info, label in zip(video_infos, labels):
+            print(info, video_filename)
+            assert info['filename'] == video_filename
+            assert set(info['label']) == set(label)
+        assert video_dataset.start_index == 0
+
     def test_video_pipeline(self):
         target_keys = ['filename', 'label', 'start_index', 'modality']