More support for pose estimation in yolo-v8. (#599)

* More support for pose estimation in yolo-v8.

* Support both object detection and pose-estimation in the yolo-v8 example.

candle-examples/examples/yolo-v3/main.rs

@@ -65,6 +65,7 @@ pub fn report(
                     xmax: pred[0] + pred[2] / 2.,
                     ymax: pred[1] + pred[3] / 2.,
                     confidence,
+                    keypoints: vec![],
                 };
                 bboxes[class_index].push(bbox)
             }