Merge pull request #5036 from microsoft/promote-retiarii-to-nas

[DO NOT SQUASH] Promote retiarii to NAS

nni/nas/execution/common/__init__.py

+# Copyright (c) Microsoft Corporation.
+# Licensed under the MIT license.
+
+from .engine import *
+from .graph_op import *
+from .graph import *
+from .integration_api import *
+from .integration import *
+from .listener import *
+from .utils import *

nni/algorithms/nas/pytorch/pdarts/__init__.py → nni/nas/execution/pytorch/cgo/__init__.py

 # Copyright (c) Microsoft Corporation.
 # Licensed under the MIT license.
 
-from .trainer import PdartsTrainer
+from .engine import *

nni/algorithms/nas/pytorch/cream/__init__.py → nni/nas/execution/pytorch/converter/__init__.py

 # Copyright (c) Microsoft Corporation.
 # Licensed under the MIT license.
 
-from .trainer import CreamSupernetTrainer
+from .graph_gen import convert_to_graph