make Registry default factory to Registr

Signed-off-by: Zhiyuan Chen <[email protected]>

chanfig/registry.py

@@ -108,6 +108,7 @@ def __init__(
             self.setattr("key", key)
         if default is not None:
             self.setattr("default", default)
+        self.setattr("default_factory", Registry)
 
     def register(
         self, component: Any = Null, name: Any = Null, override: bool = False, default: bool = False
@@ -201,7 +202,10 @@ def lookup(self, name: str, default: Any = Null) -> Any:
 
         if default is Null:
             default = self.getattr("default", Null)
-        return self.get(name, default)
+        element = self.get(name, default)
+        if isinstance(element, Registry):
+            return element.getattr("default")
+        return element
 
     @staticmethod
     def init(cls: Callable, *args: Any, **kwargs: Any) -> Any:  # pylint: disable=W0211