Add compatibility with FFmpeg 8.0

key_frame was deprecated in 2023 (FFmpeg 6.1):

2023-05-04 - 0fc9c1f6828 - lavu 58.7.100 - frame.h
  Deprecate AVFrame.interlaced_frame, AVFrame.top_field_first, and
  AVFrame.key_frame.
  Add AV_FRAME_FLAG_INTERLACED, AV_FRAME_FLAG_TOP_FIELD_FIRST, and
  AV_FRAME_FLAG_KEY flags as replacement.

Author: rathann

torchvision/csrc/io/decoder/video_stream.cpp

@@ -122,7 +122,11 @@ int VideoStream::copyFrameBytes(ByteStorage* out, bool flush) {
 void VideoStream::setHeader(DecoderHeader* header, bool flush) {
   Stream::setHeader(header, flush);
   if (!flush) { // no frames for video flush
+#if LIBAVUTIL_VERSION_INT >= AV_VERSION_INT(58,7,100)
+    header->keyFrame = (frame_)->flags & AV_FRAME_FLAG_KEY;
+#else
     header->keyFrame = frame_->key_frame;
+#endif
     header->fps = av_q2d(av_guess_frame_rate(
         inputCtx_, inputCtx_->streams[format_.stream], nullptr));
   }