refine generate (#1562)

### What problem does this PR solve?



### Type of change

- [x] Refactoring

graph/component/base.py

@@ -445,6 +445,11 @@ 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.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:
+                    upstream_outs.append(o)
+                    continue
             if u not in self._canvas.get_component(self._id)["upstream"]: continue
             if self.component_name.lower().find("switch") < 0 \
                     and self.get_component_name(u) in ["relevant", "categorize"]:

graph/component/generate.py

@@ -72,7 +72,7 @@ class Generate(ComponentBase):
         prompt = self._param.prompt
 
         retrieval_res = self.get_input()
-        input = "\n- ".join(retrieval_res["content"])
+        input = "\n- ".join(retrieval_res["content"]) if "content" in retrieval_res else ""
         for para in self._param.parameters:
             cpn = self._canvas.get_component(para["component_id"])["obj"]
             _, out = cpn.output(allow_partial=False)