Change ModelCheckpoint.save_best_only without changing any other behaviour
How do I change the default behaviour ModelCheckpoint.save_best_only=False to True without changing any other behaviour, like where the checkpoints should be stored, or logging dir etc. When creating my own ModelCheckpoint like described in the docs, will delete the os.getcwd() directory and put the checkpoint there:

# DEFAULTS used by the Trainer
checkpoint_callback = ModelCheckpoint(
    filepath=os.getcwd(),
    save_best_only=True,
    verbose=True,
    monitor='val_loss',
    mode='min',
    prefix=''
)

trainer = Trainer(checkpoint_callback=checkpoint_callback)

I have no Idea how to get the actual logging dir from the trainer, as I have to instantiate the Trainer with the callback. Couldn't find any easy nice solution without doing some nasty stuff. Can you help me here?
This seems not to work either..
     log_dir = "/log"
    checkpoint_callback = ModelCheckpoint(
        filepath=log_dir + "/checkpoints",
        save_best_only=True,
        verbose=True,
        monitor="val_acc",
        mode="min",
        prefix="",
    )

    trainer = Trainer(
        gpus=1,
        checkpoint_callback=checkpoint_callback,
        default_save_path=log_dir,
        train_percent_check=0.1,
    )

In my opinion that's a really bad default behaviour as the drive will blow up saving a checkpoint each epoch.
What's your environment?

python 3.7.4
torch 1.3.0
Lightning 0.5.2.1
Test-tube 0.7.1