Adapt has_labels test when no labels were found (#20113)

* Make default labels for non-pretrained models

* Fix the has_labels test instead

src/transformers/trainer.py

@@ -3189,7 +3189,7 @@ class Trainer:
             Tuple[Optional[torch.Tensor], Optional[torch.Tensor], Optional[torch.Tensor]]: A tuple with the loss,
             logits and labels (each being optional).
         """
-        has_labels = all(inputs.get(k) is not None for k in self.label_names)
+        has_labels = False if len(self.label_names) == 0 else all(inputs.get(k) is not None for k in self.label_names)
         inputs = self._prepare_inputs(inputs)
         if ignore_keys is None:
             if hasattr(self.model, "config"):