cb_2.py

import pytorch_lightning as pl

class MyLoggingCallback(pl.Callback):

    def on_init_start(self, trainer):
        trainer.logger.experiment.log_tensorboard_images(...)

    def on_init_end(self, trainer):
        trainer.logger.experiment.save_or_something(...)

    def on_train_end(self, trainer, pl_module):
        trainer.logger.experiment.log_something_else(...)

# pass to trainer
trainer = pl.Trainer(callbacks=[MyPrintingCallback()])