Compute balanced accuracy as additional score. Useful for imbalanced data. Only implemented for model with mutually exclusive targets.

## Examples

```
if (FALSE) { # reticulate::py_module_available("tensorflow")
y_true <- c(1,0,0,1,
0,1,0,0,
0,0,1,0) %>% matrix(ncol = 3)
y_pred <- c(0.9,0.1,0.2,0.1,
0.05,0.7,0.2,0.0,
0.05,0.2,0.6,0.9) %>% matrix(ncol = 3)
cm_dir <- tempfile()
dir.create(cm_dir)
bal_acc_metric <- balanced_acc_wrapper(num_targets = 3L, cm_dir = cm_dir)
bal_acc_metric$update_state(y_true, y_pred)
bal_acc_metric$result()
as.array(bal_acc_metric$cm)
}
```