control Tensor-Float-32 evaluation in RNNs.
    *   Added a 'experimental_payloads' field to tf.errors.OpError and its
        subclasses to support more detailed error reporting. This is inspired
        from Abseil Status payloads:
        https://github.com/abseil/abseil-cpp/blob/master/absl/status/status.h

*   `tf.summary`:

    *   New `tf.summary.graph` allows manual write of TensorFlow graph