*   `Estimator` now supports `Dataset`: `input_fn` can return a `Dataset`
        instead of `Tensor`s.
    *   Add `RevBlock`, a memory-efficient implementation of reversible residual
        layers.
    *   Reduce BFCAllocator internal fragmentation.
    *   Add `cross_entropy` and `kl_divergence` to
        `tf.distributions.Distribution`.