Callback closure for restetting the booster's parameters at each iteration.
cb.reset.parameters(new_params)a list where each element corresponds to a parameter that needs to be reset.
Each element's value must be either a vector of values of length nrounds
to be set at each iteration,
or a function of two parameters learning_rates(iteration, nrounds)
which returns a new parameter value by using the current iteration number
and the total number of boosting rounds.
This is a "pre-iteration" callback function used to reset booster's parameters at the beginning of each iteration.
Note that when training is resumed from some previous model, and a function is used to
reset a parameter value, the nround argument in this function would be the
the number of boosting rounds in the current training.
Callback function expects the following values to be set in its calling frame:
bst or bst_folds,
iteration,
begin_iteration,
end_iteration.