save_function() not set with save_model callback?
This is the callback in trainer()
trainer = pl.Trainer(
        callbacks=[ModelCheckpoint(monitor='val_loss',
filepath=os.path.join(hparams.default_root_dir,
'{epoch}-{val_loss:.2f}-{test_acc:.2f}'), verbose=True) ],

But the app crashes on the first epoch on the following error
Exception has occurred: ValueError
.save_function() not set
  File "/home/AAA/anaconda3/envs/BBB/lib/python3.7/site-packages/pytorch_lightning/callbacks/model_checkpoint.py", line 133, in _save_model
    raise ValueError(".save_function() not set")
  File "/home/AAA/anaconda3/envs/BBB/lib/python3.7/site-packages/pytorch_lightning/callbacks/model_checkpoint.py", line 240, in _do_check_save
    self._save_model(filepath)
  File "/home/AAA/anaconda3/envs/BBB/lib/python3.7/site-packages/pytorch_lightning/callbacks/model_checkpoint.py", line 208, in on_validation_end
    self._do_check_save(filepath, current, epoch)
  File "/home/AAA/anaconda3/envs/BBB/lib/python3.7/site-packages/pytorch_lightning/trainer/callback_hook.py", line 63, in on_validation_end
    callback.on_validation_end(self, self.get_model())
  File "/home/AAA/anaconda3/envs/BBB/lib/python3.7/site-packages/pytorch_lightning/trainer/training_loop.py", line 792, in call_checkpoint_callback
    self.on_validation_end()
  File "/home/AAA/anaconda3/envs/BBB/lib/python3.7/site-packages/pytorch_lightning/trainer/training_loop.py", line 477, in run_training_epoch
    self.call_checkpoint_callback()
  File "/home/AAA/anaconda3/envs/BBB/lib/python3.7/site-packages/pytorch_lightning/trainer/training_loop.py", line 363, in train
    self.run_training_epoch()
  File "/home/AAA/anaconda3/envs/BBB/lib/python3.7/site-packages/pytorch_lightning/trainer/trainer.py", line 865, in run_pretrain_routine
    self.train()
  File "/home/AAA/anaconda3/envs/BBB/lib/python3.7/site-packages/pytorch_lightning/trainer/distrib_parts.py", line 477, in single_gpu_train
    self.run_pretrain_routine(model)
  File "/home/AAA/anaconda3/envs/BBB/lib/python3.7/site-packages/pytorch_lightning/trainer/trainer.py", line 705, in fit
    self.single_gpu_train(model)
  File "/home/AAA/PycharmProjects/DL2020LiorWolf/train.py", line 110, in main_train
    trainer.fit(model)
  File "/home/AAA/PycharmProjects/DL2020LiorWolf/train.py", line 40, in main
    main_train(model_class_pointer, hyperparams, logger)
  File "/home/AAA/PycharmProjects/DL2020LiorWolf/train.py", line 118, in <module>
    main()
  File "/home/AAA/anaconda3/envs/BBB/lib/python3.7/runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "/home/AAA/anaconda3/envs/BBB/lib/python3.7/runpy.py", line 96, in _run_module_code
    mod_name, mod_spec, pkg_name, script_name)
  File "/home/AAA/anaconda3/envs/BBB/lib/python3.7/runpy.py", line 263, in run_path
    pkg_name=pkg_name, script_name=fname)
  File "/home/AAA/anaconda3/envs/BBB/lib/python3.7/runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "/home/AAA/anaconda3/envs/BBB/lib/python3.7/runpy.py", line 193, in _run_module_as_main
    "__main__", mod_spec)

From the docs the model_checkpoint module seems as a "plug-and-play", I need to implement something else?
Actually, going through the source code, it seems as save_function is never set