*   Add meta-distribution BatchReshape which reshapes batch dimensions.
    *   `tf.contrib.layers.recompute_grad` works for explicit gradient
        checkpointing on TPU.
    *   Add `tf.contrib.framework.argsort`.
    *   Allow `DNNBoostedTreeCombinedEstimator` to work with core versions of
        feature columns and losses.
    *   Add non-linear image warping ops: `tf.contrib.image.sparse_image_warp`,