[booster] Implement CheckpointIO for Native PyTorch

[FrankLeeeee]

Overview

CheckpointIO takes care of the Booster.save and Booster.load logic to allow for model saving/resuming/loading. It should be noted that CheckpointIO is often used in pair with the Plugin as a Plugin can possibly require a specific saving/loading strategy. However, we should propose general ones for normal pytorch model and a DTensor-based model. As the DTensor is under development, we should focus on the native PyTorch implementation first.

Wanna track the development progress? Take a look at

proposal: #3046
project kanban: https://github.com/orgs/hpcaitech/projects/19

Goal

The CheckpointIO should allow the user to save/load the native PyTorch model/optimizer/lr schduler.