Merge pull request #26142 from danaugrs:patch2

PiperOrigin-RevId: 237336877

tensorflow/python/keras/optimizer_v2/optimizer_v2.py

@@ -358,6 +358,7 @@ class OptimizerV2(trackable.Trackable):
       ValueError: In case any gradient cannot be computed (e.g. if gradient
         function not implemented).
     """
+    with backend.get_graph().as_default():
       grads = gradients.gradients(loss, params)
     if None in grads:
       raise ValueError("An operation has `None` for gradient. "