Callback closure for saving a model file.
cb.save.model(save_period = 0, save_name = "xgboost.model")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_
the file saved at iteration 50 would be named "xgboost_0050.model".
This callback function allows to save an xgb-model file, either periodically after each save_period's or at the end.
Callback function expects the following values to be set in its calling frame:
bst,
iteration,
begin_iteration,
end_iteration.