read_video: accept BytesIO on pyav backend (#8442)

Author: NicolasHug

torchvision/io/video.py

@@ -244,7 +244,7 @@ def read_video(
     Reads a video from a file, returning both the video frames and the audio frames
 
     Args:
-        filename (str): path to the video file
+        filename (str): path to the video file. If using the pyav backend, this can be whatever ``av.open`` accepts.
         start_pts (int if pts_unit = 'pts', float / Fraction if pts_unit = 'sec', optional):
             The start presentation time of the video
         end_pts (int if pts_unit = 'pts', float / Fraction if pts_unit = 'sec', optional):
@@ -267,10 +267,9 @@ def read_video(
 
     from torchvision import get_video_backend
 
-    if not os.path.exists(filename):
-        raise RuntimeError(f"File not found: {filename}")
-
     if get_video_backend() != "pyav":
+        if not os.path.exists(filename):
+            raise RuntimeError(f"File not found: {filename}")
         vframes, aframes, info = _video_opt._read_video(filename, start_pts, end_pts, pts_unit)
     else:
         _check_av_available()