*   XLA:
    *   Add a new API that allows custom call functions to signal errors. The
        old API will be deprecated in a future release. See
        https://www.tensorflow.org/xla/custom_call for details.
    *   XLA:GPU reductions are deterministic by default (reductions within
        `jit_compile=True` are now deterministic).
    *   XLA:GPU works with Horovod (OSS contribution by Trent Lo from NVidia)