assign role_maker before use (#18137)

fix role_maker bug
test=develop

python/paddle/fluid/incubate/fleet/base/fleet_base.py

@@ -188,6 +188,7 @@ class Fleet(object):
         if role_maker and not isinstance(role_maker, RoleMakerBase):
             raise ValueError("role_maker must be an instance of RoleMakerBase")
 
+        self._role_maker = role_maker
         self._role_maker.generate_role()
 
         self._is_initialized = True