Write to tensorboard in Rust

Write TensorBoard events in Rust.

• Can write scalar, image, histogram.

Example

• Write multiple scalar in one plot.

    let mut writer = SummaryWriter::new(&("./logdir".to_string()));

    for n_iter in 0..100 {
        let mut map = HashMap::new();
        map.insert("x1".to_string(), (n_iter as f32));
        map.insert("x^2".to_string(), (n_iter as f32) * (n_iter as f32));
        writer.add_scalars("data/scalar_group", &map, n_iter);
    }
    writer.flush();