Add tracing to segment-anything (#777)

* Tracing support for segment-anything.

* More tracing.

* Handle the empty slice case.

candle-examples/examples/whisper/main.rs

@@ -431,7 +431,6 @@ fn main() -> Result<()> {
 
     let args = Args::parse();
     let _guard = if args.tracing {
-        println!("tracing...");
         let (chrome_layer, guard) = ChromeLayerBuilder::new().build();
         tracing_subscriber::registry().with(chrome_layer).init();
         Some(guard)