Return Prediction Mapping Function (Helper)
retpred_map(t, data_subset, indices, model_function, model_config)
integer, index of the current iteration
tibble, subset of the data (stock_id, date, return_label, features)
list, list of training and prediction indices
character, name of the prediction (model) function
list, configuration for the model
tibble that contains stock id, date, and predicted return