Model checkpoint is not working
Describe the bug
Model checkpoint is not working, even with explicit checkpoint callback.
To Reproduce
Steps to reproduce the behavior:
This is the settings I'm using. hparams.checkpoint_path is actually a dir like './weights'
checkpoint_callback = ModelCheckpoint(
        filepath=hparams.checkpoint_path,
        save_best_only=True,
        verbose=True,
        monitor='val_loss',
        mode='min',
        prefix='')

    model = Net3DMMSTN(hparams)
    trainer = Trainer(
        early_stop_callback=None,
        track_grad_norm=2,
        checkpoint_callback=checkpoint_callback,
        print_nan_grads=True,
        weights_summary='full',
        default_save_path=hparams.checkpoint_path,
        max_nb_epochs=hparams.max_nb_epochs,
        gpus=hparams.gpus,
        fast_dev_run=hparams.dev_run)

It's not saving the model weights.
Even leaving the checkpoint_callback in default mode it' not saving the weights.
Expected behavior
A clear and concise description of what you expected to happen.
Screenshots
If applicable, add screenshots to help explain your problem.
Desktop (please complete the following information):

OS: Linux
os kernel version: 21~18.04.1-Ubuntu SMP Mon Oct 7 04:51:28 UTC 2019
os release version: 5.0.0-1021-gcp
os platform: Linux-5.0.0-1021-gcp-x86_64-with-Ubuntu-18.04-bionic
pytorch-lightning : 0.5.3
PyTorch Version : 1.3.0
== cuda libs  ===================================================
/usr/local/cuda-10.0/doc/man/man7/libcudart.7
/usr/local/cuda-10.0/doc/man/man7/libcudart.so.7
/usr/local/cuda-10.0/lib64/libcudart_static.a
/usr/local/cuda-10.0/lib64/libcudart.so.10.0.130