No way to disable checkpoint callback
With the recent change to provide a checkpoint callback by default, there's no way to disable checkpointing. Passing checkpoint_callback=None results in the default checkpointer being created.
In Trainer.__Init__:
        self.checkpoint_callback = checkpoint_callback
        if self.checkpoint_callback is None:
            if isinstance(logger, TestTubeLogger):
                ckpt_path = '{}/{}/{}'.format(self.default_save_path, self.logger.name,
                                              self.logger.version)
            else:
                ckpt_path = self.default_save_path

            self.checkpoint_callback = ModelCheckpoint(
                filepath=ckpt_path
            )