[fbsync] Add missing doc for get_video_backend (#3643)

Summary:

Reviewed By: NicolasHug

Differential Revision: D27706956

fbshipit-source-id: c5e3f4030b9df7081d72ea9a2e307cadb9f0a676

Co-authored-by: Nicolas Hug <[email protected]>
Co-authored-by: Nicolas Hug <[email protected]>

Author: 2 people

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