fix UCF101 on Windows (#5129)

Author: pmeier

test/test_datasets.py

@@ -854,7 +854,7 @@ def _annotation_file_name(self, fold, train):
 
     def _create_annotation_file(self, root, name, video_files):
         with open(pathlib.Path(root) / name, "w") as fh:
-            fh.writelines(f"{file}\n" for file in sorted(video_files))
+            fh.writelines(f"{str(file).replace(os.sep, '/')}\n" for file in sorted(video_files))
 
 
 class LSUNTestCase(datasets_utils.ImageDatasetTestCase):

torchvision/datasets/ucf101.py

@@ -108,7 +108,7 @@ def _select_fold(self, video_list: List[str], annotation_path: str, fold: int, t
         with open(f) as fid:
             data = fid.readlines()
             data = [x.strip().split(" ")[0] for x in data]
-            data = [os.path.join(self.root, x) for x in data]
+            data = [os.path.join(self.root, *x.split("/")) for x in data]
             selected_files.update(data)
         indices = [i for i in range(len(video_list)) if video_list[i] in selected_files]
         return indices