[booster] fix pretrain dataloader method of plugin

[ver217]

Overview

DPPluginBase class has a method named prepare_train_dataloader(), which prepares a dataloader for distributed training from
a dataset. This method should be general and be defined in the base class Plugin.

Wanna track the development progress? Take a look at

proposal: #3046
project kanban: API Refactoring

Goal

1. Define prepare_train_dataloader() method in Plugin class.
2. Rename this method to prepare_dataloader().
3. Update corresponding examples.