Fix random_activation_in_scope panic and simplify implementation

src/activation.rs

@@ -93,24 +93,24 @@ impl ActivationRegistry {
     }
 
     /// Fetches a random activation fn that applies to the provided scope.
+    ///
+    /// # Panics
+    ///
+    /// Panics if there are no activation functions registered for the given scope.
     pub fn random_activation_in_scope(
         &self,
         scope: NeuronScope,
         rng: &mut impl rand::Rng,
     ) -> ActivationFn {
-        let mut iter = self.fns.values().cycle();
-        let num_iterations = rng.random_range(0..self.fns.len() - 1);
+        let activations = self.activations_in_scope(scope);
 
-        for _ in 0..num_iterations {
-            iter.next().unwrap();
-        }
-
-        let mut val = iter.next().unwrap();
-        while !val.scope.contains(scope) {
-            val = iter.next().unwrap();
-        }
+        assert!(
+            !activations.is_empty(),
+            "no activation functions registered for scope {:?}",
+            scope
+        );
 
-        val.clone()
+        activations[rng.random_range(0..activations.len())].clone()
     }
 }