Fix missing RequestLoader return types

Author: sjmonson

src/guidellm/request/loader.py

@@ -30,10 +30,10 @@ class RequestLoaderDescription(StandardBaseModel):
 
 class RequestLoader(Iterable):
     @abstractmethod
-    def __iter__(self): ...
+    def __iter__(self) -> Iterator: ...
 
     @abstractmethod
-    def __len__(self): ...
+    def __len__(self) -> int: ...
 
     @property
     @abstractmethod