Fix JSON serialization error in TensorFlowOpLayer in Python 3.

PiperOrigin-RevId: 259397921

tensorflow/python/keras/engine/base_layer.py

@@ -2387,6 +2387,8 @@ def __init__(self,
                dtype=None):
     super(TensorFlowOpLayer, self).__init__(
         name=_TF_OP_LAYER_NAME_PREFIX + name, trainable=trainable, dtype=dtype)
+    if not isinstance(node_def, bytes):
+      node_def = node_def.encode('utf-8')
     self.node_def = node_def_pb2.NodeDef.FromString(node_def)
     self.constants = constants or {}
     # Layer uses original op unless it is called on new inputs.
@@ -2446,7 +2448,7 @@ def _defun_call(self, inputs):
   def get_config(self):
     config = super(TensorFlowOpLayer, self).get_config()
     config.update({
-        'node_def': self.node_def.SerializeToString(),
+        'node_def': self.node_def.SerializeToString().decode('utf-8'),
         'constants': {
             i: backend.get_value(c) for i, c in self.constants.items()
         }