Fix: debug pipeline... (#10311)

### What problem does this PR solve?

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

rag/flow/hierarchical_merger/hierarchical_merger.py

@@ -53,6 +53,7 @@ class HierarchicalMerger(ProcessBase):
             self.set_output("_ERROR", f"Input error: {str(e)}")
             return
 
+        self.set_output("output_format", "chunks")
         self.callback(random.randint(1, 5) / 100.0, "Start to merge hierarchically.")
         if from_upstream.output_format in ["markdown", "text", "html"]:
             if from_upstream.output_format == "markdown":