make adam_w import _C_ops from global (#40960)

python/paddle/optimizer/adamw.py

@@ -19,10 +19,9 @@ from ..fluid import framework
 from ..fluid.framework import Variable
 from ..fluid.dygraph import base as imperative_base
 from collections.abc import Callable
+from .. import _C_ops
 import paddle
 
-_C_ops = core.ops
-
 __all__ = []