Created by: seiriosPlus

First: add self.checkpoint to Train.

Second: add checkpoint feature in Train class.

class CheckpointConfig(object):
    def __init__(self, checkpoint_dir=None, max_num_checkpoints=3, save_interval_secs=600):
        if checkpoint_dir is None:
            self.checkpoint_dir = os.getcwd()
        self.max_num_checkpoints = max_num_checkpoints
        self.save_interval_secs = save_interval_secs

Thrid: add checkpoint_config in Train init. Add load_checkpoint in Train init. Add save_checkpoint in Train _train_by_any_executor. Add clean_checkpoint in Train _train_by_any_executor