*   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`:

mean to be reused in gradient computation. When is_training is
False, a 1D Tensor for the population mean to be reused in both
1st and 2nd order gradient computation.}]>:$reserve_space_1,
    Arg<TF_Float32Tensor, [{When is_training is True, a 1D Tensor for the computed batch
variance (inverted variance in the cuDNN case) to be reused in
gradient computation. When is_training is False, a 1D Tensor