Stores checkpoint to a file.

        :param identifier: identifier for periodic checkpoint
        :param is_best: if True stores checkpoint to 'model_best.pth'
        :param save_all: if True stores checkpoint after completed training
            epoch

def save(self, identifier=None, is_best=False, save_all=False):
        """
        Stores checkpoint to a file.

        :param identifier: identifier for periodic checkpoint
        :param is_best: if True stores checkpoint to 'model_best.pth'
        :param save_all: if True stores checkpoint after completed training
            epoch
        """

        def write_checkpoint(state, filename):
            filename = os.path.join(self.save_path, filename)
            logging.info(f'Saving model to {filename}')
            torch.save(state, filename)

        if self.distributed:
            model_state = self.model.module.state_dict()
        else:
            model_state = self.model.state_dict()

        state = {
            'epoch': self.epoch,
            'state_dict': model_state,
            'optimizer': self.optimizer.state_dict(),
            'scheduler': self.scheduler.state_dict(),
            'loss': getattr(self, 'loss', None),
        }
        state = dict(list(state.items()) + list(self.save_info.items()))

        if identifier is not None:
            filename = self.checkpoint_filename % identifier
            write_checkpoint(state, filename)

        if is_best:
            filename = 'model_best.pth'
            write_checkpoint(state, filename)

        if save_all:
            filename = f'checkpoint_epoch_{self.epoch:03d}.pth'
            write_checkpoint(state, filename)