behavior.
*   Added None check for save_path in `saver.restore`.
*   Register devices under their legacy names in device_mgr to ease the
    transition to clusterspec-propagated configurations.
*   VectorExponential added to distributions.
*   Add a bitwise module with bitwise_and, bitwise_or, bitwise_xor, and invert
    functions.
*   Add fixed-grid ODE integration routines.