0.10.0   checkpoint ValueError: .save_function() not set 
This is returned value in my validation_step:
        self.log('val_loss', loss, on_epoch=True, sync_dist=True)
        self.log('val_pacc', part_acc, on_epoch=True, sync_dist=True)
        self.log('val_acc', vid_acc, on_epoch=True, prog_bar=True, sync_dist=True)



        return {'val_loss': loss, "val_pacc": part_acc, "val_acc": vid_acc}

This is my checkpoint callback:
    checkpoint_callback = ModelCheckpoint(
        monitor='val_acc',
        mode='max',
    )

training is ok, but in validation step, it throws error, shows
  File "/mnt/lustre/maxiao1/anaconda3/lib/python3.7/site-packages/pytorch_lightning/callbacks/model_checkpoint.py", line 297, in _save_model
    raise ValueError(".save_function() not set")
ValueError: .save_function() not set