Callback closure for logging the evaluation history
cb.evaluation.log()This callback function appends the current iteration evaluation results bst_evaluation
available in the calling parent frame to the evaluation_log list in a calling frame.
The finalizer callback (called with finalize = TURE in the end) converts
the evaluation_log list into a final data.table.
The iteration evaluation result bst_evaluation must be a named numeric vector.
Note: in the column names of the final data.table, the dash '-' character is replaced with the underscore '_' in order to make the column names more like regular R identifiers.
Callback function expects the following values to be set in its calling frame:
evaluation_log,
bst_evaluation,
iteration.