let 'Generate' take user's input as parameter (#3086)

### What problem does this PR solve?

#3085

### Type of change

- [x] New Feature (non-breaking change which adds functionality)

agent/canvas.py

@@ -260,7 +260,7 @@ class Canvas(ABC):
 
     def get_history(self, window_size):
         convs = []
-        for role, obj in self.history[(window_size + 1) * -1:]:
+        for role, obj in self.history[window_size * -1:]:
             convs.append({"role": role, "content": (obj if role == "user" else
                     '\n'.join([str(s) for s in pd.DataFrame(obj)['content']]))})
         return convs

agent/component/generate.py

@@ -105,6 +105,9 @@ class Generate(ComponentBase):
         input = ("  - "+"\n  - ".join([c for c in retrieval_res["content"] if isinstance(c, str)])) if "content" in retrieval_res else ""
         for para in self._param.parameters:
             cpn = self._canvas.get_component(para["component_id"])["obj"]
+            if cpn.component_name.lower() == "answer":
+                kwargs[para["key"]] = self._canvas.get_history(1)[0]["content"]
+                continue
             _, out = cpn.output(allow_partial=False)
             if "content" not in out.columns:
                 kwargs[para["key"]] = "Nothing"