Add missing to_list to multi_gpu_utils (#10795)

keras/utils/multi_gpu_utils.py

@@ -9,6 +9,7 @@ from .. import backend as K
 from ..layers.core import Lambda
 from ..engine.training import Model
 from ..models import clone_model
+from ..utils.generic_utils import to_list
 
 
 def _get_available_devices():