Update ucf101.py (#2186)
Update ucf101.py (#2186)

Now the dataset is not working properly because of this line of code `indices = [i for i in range(len(video_list)) if video_list[i][len(self.root) + 1:] in selected_files]`. 
Performing the `len(self.root) + 1` only make sense if there is no training / to root

```
>>> root = 'data/ucf-101/videos'
>>> video_path = 'data/ucf-101/videos/activity/video.avi'
>>> video_path [len(root ):]
'/activity/video.avi'
>>> video_path [len(root ) + 1:]
'activity/video.avi'
```

Appending the root path also to the selected files is a simple solution and make the dataset works with and without a trailing slash.
pytorch/vision,pytorch/vision,pytorch/vision,pytorch/vision,pytorch/vision,pytorch/vision