Unify the API for all models to make predictions on the test set

[WenjieDu]

1. Feature description

Currently, we use different APIs for models on different tasks to make predictions, e.g. impute() for imputation models, classify() for classification models, cluster() for clustering ones, and forecast() for forecasting ones. We should make a unified API for all models to make predictions (including all tasks) on the given test set.

2. Motivation

Make the framework's API more unified and easier to use.

3. Your contribution

Will make a PR to implement it.