This reverts commit d3941dc1.

This reverts commit 32593b63.

This reverts commit 9beb154b.

This reverts commit 58daf805.

This reverts commit 428edb6f.

This reverts commit 9e683d0d.

1. ModelSaver();
2. _build_ema_model();
3. _init_checkpoints();
4. others.

1. rm engine.device and use paddle.device.get_device() instead;
2. mv some code to build_train_func or build_eval_func to simpfy engine.