[pipeline] refactor the pipeline module (#1087)

* [pipeline] refactor the pipeline module

* polish code

colossalai/nn/__init__.py

@@ -2,6 +2,5 @@ from .layer import *
 from .loss import *
 from .lr_scheduler import *
 from .metric import *
-from .model import *
 from .optimizer import *
 from ._ops import *