*   Normalization can now accept mean and variance values as init args.
*   In `Attention` and `AdditiveAttention` layers, the `call()` method now
    accepts a `return_attention_scores` argument. When set to True, the layer
    returns the attention scores as an additional output argument.
*   Adds `tf.metrics.log_cosh` and `tf.metrics.logcosh` API entrypoints with the
    same implementation as their `tf.losses` equivalent.