[CoreEngine] add the missed import.

Author: fedml-alex

python/fedml/computing/scheduler/scheduler_core/compute_gpu_db.py

@@ -8,6 +8,7 @@
 from fedml.core.common.singleton import Singleton
 from .base_db import FedMLBaseDb
 from .compute_utils import ComputeUtils
+from ..master.server_constants import ServerConstants
 
 Base = declarative_base()