feat: raise initializer error on unavailable data

prototorch/core/initializers.py

@@ -217,6 +217,8 @@ class AbstractStratifiedCompInitializer(AbstractClassAwareCompInitializer):
         components = torch.tensor([])
         for k, v in distribution.items():
             stratified_data = self.data[self.targets == k]
+            if len(stratified_data) == 0:
+              raise ValueError(f"No data available for class {k}.")
             initializer = self.subinit_type(
                 stratified_data,
                 noise=self.noise,