*   Improves stability of GCS filesystem on Mac.
*   Add time estimation to HloCostAnalysis.
*   Fixed the bug in Estimator that params in constructor was not a deepcopy of
    the user provided one. This bugs inadvertently enabled user to mutate the
    params after the creation of Estimator, leading to potentially undefined
    behavior.
*   Added None check for save_path in `saver.restore`.