Add missing doc for get_video_backend (#3643)

Co-authored-by: Nicolas Hug <contact@nicolas-hug.com>
Co-authored-by: Nicolas Hug <nicolashug@fb.com>

torchvision/__init__.py

@@ -85,6 +85,13 @@ def set_video_backend(backend):
 
 
 def get_video_backend():
+    """
+    Returns the currently active video backend used to decode videos.
+
+    Returns:
+        str: Name of the video backend. one of {'pyav', 'video_reader'}.
+    """
+
     return _video_backend