reorgnize colotensor directory (#1062)

* reorgnize colotensor directory

* polish code

colossalai/nn/__init__.py

@@ -4,3 +4,7 @@ from .lr_scheduler import *
 from .metric import *
 from .model import *
 from .optimizer import *
+from ._ops import *
+
+from .modules import ColoLinear, ColoEmbedding
+from .module_utils import register_colo_module, is_colo_module, get_colo_module, init_colo_module, check_colo_module