Allow the threshold argumet to be negative in the segment-anything example (#2187)

Threshold is 0.0 by default, negative values make more points included,
expanding the mask. Positive values make it more picky, making the mask
smaller.

Negative numbers start with a minus sign, which normally makes clap
consider it a flag.

candle-examples/examples/segment-anything/main.rs

@@ -39,7 +39,7 @@ struct Args {
 
     /// The detection threshold for the mask, 0 is the default value, negative values mean a larger
     /// mask, positive makes the mask more selective.
-    #[arg(long, default_value_t = 0.)]
+    #[arg(long, allow_hyphen_values = true, default_value_t = 0.)]
     threshold: f32,
 
     /// Enable tracing (generates a trace-timestamp.json file).