Add yolo v8 as an example (#541)

* Sketching yolo-v8.

* Get the model to load.

* yolo-v8 forward pass.

* Complete(?) the forward pass.

* Fix some shape issues.

* Add the missing padding.

* Process the predictions.

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

@@ -199,6 +199,7 @@ pub fn main() -> Result<()> {
         };
         let image = (image.unsqueeze(0)?.to_dtype(DType::F32)? * (1. / 255.))?;
         let predictions = model.forward(&image)?.squeeze(0)?;
+        println!("generated predictions {predictions:?}");
         let image = report(&predictions, original_image, net_width, net_height)?;
         image_name.set_extension("pp.jpg");
         println!("writing {image_name:?}");