Unable to save multi-task recommender model

I was following the guide for the multi-task recommender found here, but when I tried to save using model.save(), I was unable to to do so with the following error:

WARNING:tensorflow:Skipping full serialization of Keras layer <tensorflow_recommenders.metrics.factorized_top_k.FactorizedTopK object at 0x7fa43148ae80>, because it is not built.
WARNING:tensorflow:Skipping full serialization of Keras layer <tensorflow_recommenders.metrics.factorized_top_k.FactorizedTopK object at 0x7fa43148ae80>, because it is not built.
WARNING:tensorflow:From /usr/local/lib/python3.6/dist-packages/tensorflow/python/training/tracking/tracking.py:111: Model.state_updates (from tensorflow.python.keras.engine.training) is deprecated and will be removed in a future version.
Instructions for updating:
This property should not be used in TensorFlow 2.0, as updates are applied automatically.
WARNING:tensorflow:From /usr/local/lib/python3.6/dist-packages/tensorflow/python/training/tracking/tracking.py:111: Model.state_updates (from tensorflow.python.keras.engine.training) is deprecated and will be removed in a future version.
Instructions for updating:
This property should not be used in TensorFlow 2.0, as updates are applied automatically.
WARNING:tensorflow:From /usr/local/lib/python3.6/dist-packages/tensorflow/python/keras/engine/training.py:2309: Layer.updates (from tensorflow.python.keras.engine.base_layer) is deprecated and will be removed in a future version.
Instructions for updating:
This property should not be used in TensorFlow 2.0, as updates are applied automatically.
WARNING:tensorflow:From /usr/local/lib/python3.6/dist-packages/tensorflow/python/keras/engine/training.py:2309: Layer.updates (from tensorflow.python.keras.engine.base_layer) is deprecated and will be removed in a future version.
Instructions for updating:
This property should not be used in TensorFlow 2.0, as updates are applied automatically.
---------------------------------------------------------------------------
FailedPreconditionError                   Traceback (most recent call last)
<ipython-input-10-d6cd16d67b34> in <module>()
----> 1 model.save('./')

23 frames
/usr/local/lib/python3.6/dist-packages/six.py in raise_from(value, from_value)

FailedPreconditionError: Failed to serialize the input pipeline graph: ResourceGather is stateful. 

- [ ] 

- [ ] [Op:DatasetToGraphV2]

I also cannot save it in HDF5 format, but I believe that's because the model in question is a custom subclassing of the model class. What is the appropriate way to save the model?