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