Bring back num_batches

Author: stefanradev93

bayesflow/datasets/online_dataset.py

@@ -90,3 +90,7 @@ def __getitem__(self, item: int) -> dict[str, np.ndarray]:
             batch = self.adapter(batch, stage=self.stage)
 
         return batch
+
+    @property
+    def num_batches(self) -> int:
+        return self._num_batches