fix: work with Tensorflow < 2.1.0
fix: work with Tensorflow < 2.1.0

tf.keras.utils.register_keras_serializable was added in TF 2.1.0, so
don't rely on it being there; just decorate the class with it if it
exists.

huggingface/pytorch-transformers,huggingface/transformers,huggingface/transformers,huggingface/transformers,huggingface/transformers