correct type hinting since fn is now a generator

src/smolagents/agents.py

@@ -503,7 +503,7 @@ def run(
         # We only want the last step and want to go through the generator efficiently
         return deque(self._run(task=self.task), maxlen=1)[0]
 
-    def _run(self, task: str) -> Union[str, Generator[str, None, None]]:
+    def _run(self, task: str) -> Generator[str, None, None]:
         """
         Runs the agent. Running can be done in direct or streaming mode.
         Note: in all cases, this function is internal and should be used only in the `run` method.