Write to tensorboard in Rust

Write TensorBoard events in Rust.

• Can write scalar.

Example

• Write multiple scalar in one plot.

```rust,norun let mut writer = SummaryWriter::new(&("./logdir".tostring()));

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();

```