refine get_input (#2630)

### What problem does this PR solve?

### Type of change

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

agent/component/base.py

@@ -444,7 +444,7 @@ class ComponentBase(ABC):
 
         if DEBUG: print(self.component_name, reversed_cpnts[::-1])
         for u in reversed_cpnts[::-1]:
-            if self.get_component_name(u) in ["switch"]: continue
+            if self.get_component_name(u) in ["switch", "concentrator"]: continue
             if self.component_name.lower() == "generate" and self.get_component_name(u) == "retrieval":
                 o = self._canvas.get_component(u)["obj"].output(allow_partial=False)[1]
                 if o is not None:
@@ -472,7 +472,7 @@ class ComponentBase(ABC):
             if "content" in df:
                 df = df.drop_duplicates(subset=['content']).reset_index(drop=True)
             return df
-        return pd.DataFrame()
+        return pd.DataFrame(self._canvas.get_history(3)[-1:])
 
     def get_stream_input(self):
         reversed_cpnts = []

agent/component/concentrator.py

@@ -33,9 +33,4 @@ class Concentrator(ComponentBase, ABC):
     component_name = "Concentrator"
 
     def _run(self, history, **kwargs):
-        input = self.get_input()
-        ans = " - ".join(input["content"]) if "content" in input else ""
-        if not ans:
-            return Concentrator.be_output("")
-
-        return Concentrator.be_output(input)
+        return Concentrator.be_output("")