fix summation checking in contrastive loss

znnl/loss_functions/contrastive_loss.py

@@ -176,7 +176,7 @@ def __call__(self, inputs: np.ndarray, targets: np.ndarray) -> float:
 
         losses = self.compute_losses(inputs, targets)
 
-        if len(losses) > 1:
+        if isinstance(losses, tuple):
             return np.array([losses]).sum()
         else:
             return losses