*   Added Adafactor optimizer `tf.keras.optimizers.Adafactor`.
    *   Added `warmstart_embedding_matrix` to `tf.keras.utils`.
        This utility can be used to warmstart an embeddings matrix so you
        reuse previously-learned word embeddings when working with a new set
        of words which may include previously unseen words (the embedding
        vectors for unseen words will be randomly initialized).

*   `tf.Variable`: