*   gRPC: Propagate truncated errors (instead of returning gRPC internal
        error).
    *   Augment `parallel_interleave` to support 2 kinds of prefetching.
    *   Improved XLA support for C64-related ops log, pow, atan2, tanh.
    *   Add probabilistic convolutional layers.

## API Changes

*   Introducing `prepare_variance` boolean with default setting to False for
    backward compatibility.