Fall back to `observed_batch_size` when the `dataloader` does not know the `batch_size`. (#13188)

src/transformers/trainer.py

@@ -2203,6 +2203,9 @@ class Trainer:
             observed_batch_size = find_batch_size(inputs)
             if observed_batch_size is not None:
                 observed_num_examples += observed_batch_size
+                # For batch samplers, batch_size is not known by the dataloader in advance.
+                if batch_size is None:
+                    batch_size = observed_batch_size
 
             # Prediction step
             loss, logits, labels = self.prediction_step(model, inputs, prediction_loss_only, ignore_keys=ignore_keys)