*   Updates to "constrained_optimization" in tensorflow/contrib.
    *   boosted trees: adding pruning mode.
    *   tf.train.Checkpoint does not delete old checkpoints by default.
    *   tfdbg: Limit the total disk space occupied by dumped tensor data to 100
        GBytes. Add environment variable `TFDBG_DISK_BYTES_LIMIT` to allow
        adjustment of this upper limit.

## Thanks to our Contributors