This callback function allows to save an xgb-model file, either periodically
after each save_period's or at the end.
Does not leave any attribute in the booster.
xgb.cb.save.model(save_period = 0, save_name = "xgboost.ubj")An xgb.Callback object, which can be passed to xgb.train(),
but not to xgb.cv().
Save the model to disk after every save_period iterations;
0 means save the model at the end.
The name or path for the saved model file.
It can contain a sprintf() formatting specifier to include the integer
iteration number in the file name. E.g., with save_name = 'xgboost_%04d.model',
the file saved at iteration 50 would be named "xgboost_0050.model".