Request batches to GeneratorStep if only step in pipeline (#828)

* Add generating batches to `GeneratorStep` if unique step in the pipeline

* Remove print

src/distilabel/pipeline/base.py

@@ -1118,6 +1118,9 @@ def _manage_batch_flow(self, batch: "_Batch") -> None:
                 self._send_batch_to_step(new_batch)
             else:
                 self._request_more_batches_if_needed(step)
+        else:
+            if len(self.dag) == 1:
+                self._request_batch_from_generator(step.name)  # type: ignore
 
         self._cache()
 
@@ -1225,6 +1228,19 @@ def _request_initial_batches(self) -> None:
             )
             self._send_batch_to_step(batch)
 
+    def _request_batch_from_generator(self, step_name: str) -> None:
+        """Request a new batch to a `GeneratorStep`.
+
+        Args:
+            step_name: the name of the `GeneratorStep` to which a batch has to be requested.
+        """
+        # Get the last batch that the previous step sent to generate the next batch
+        # (next `seq_no`).
+        last_batch = self._batch_manager.get_last_batch_sent(step_name)  # type: ignore
+        if last_batch is None:
+            return
+        self._send_batch_to_step(last_batch.next_batch())
+
     def _request_more_batches_if_needed(self, step: "Step") -> None:
         """Request more batches to the predecessors steps of `step` if needed.
 
@@ -1239,17 +1255,7 @@ def _request_more_batches_if_needed(self, step: "Step") -> None:
             if previous_step_name not in self.dag.root_steps:
                 continue
 
-            # Get the last batch that the previous step sent to generate the next batch
-            # (next `seq_no`).
-            last_batch = self._batch_manager.get_last_batch_sent(previous_step_name)  # type: ignore
-            if last_batch is None:
-                continue
-
-            self._logger.debug(
-                f"Step '{step.name}' input buffer for step '{previous_step_name}' is"
-                " empty. Requesting new batch..."
-            )
-            self._send_batch_to_step(last_batch.next_batch())
+            self._request_batch_from_generator(previous_step_name)
 
     def _handle_batch_on_stop(self, batch: "_Batch") -> None:
         """Handles a batch that was received from the output queue when the pipeline was