Clarify documentation of labels_train/test usage wrt loss_train/test.

PiperOrigin-RevId: 665469121

tensorflow_privacy/privacy/privacy_tests/membership_inference_attack/data_structures.py

@@ -261,7 +261,8 @@ class AttackInputData:
   # Contains ground-truth classes. For single-label classification, classes are
   # assumed to be integers starting from 0. For multi-label classification,
   # label is assumed to be multi-hot, i.e., labels is a binary array of shape
-  # (num_examples, num_classes).
+  # (num_examples, num_classes). Additionally used to compute the loss when
+  # loss_train/test is not provided. Leave empty for non-classification models.
   labels_train: Optional[np.ndarray] = None
   labels_test: Optional[np.ndarray] = None
 
@@ -270,7 +271,7 @@ class AttackInputData:
   sample_weight_test: Optional[np.ndarray] = None
 
   # Explicitly specified loss. If provided, this is used instead of deriving
-  # loss from logits and labels
+  # loss from logits and labels.
   loss_train: Optional[np.ndarray] = None
   loss_test: Optional[np.ndarray] = None