[RFC] Deprecate callback hooks `on_init_start` and `on_init_end`

[ananthsub]

Discussed in #10677

Part of #7740

^{Originally posted by daniellepintz November 22, 2021}
These hooks are called when trainer initialization begins and ends, before the model has been set, essentially allowing the user to modify the Trainer constructor. Should we be giving the user this much control over Trainer constructor? Are there scenarios where this is needed? Or can we deprecate these hooks?

https://github.com/PyTorchLightning/pytorch-lightning/blob/338f3cf63686935355c749920b2f298f3d18a26f/pytorch_lightning/trainer/callback_hook.py#L55-L63

@carmocca @tchaton @awaelchli do you know how these hooks are used? Have you seen any examples of these being used by the community? These hooks go way, way back, but I can't think of when they'd be needed given the user handles the Trainer initialization. It's also unclear when on_init_start actually happens: does that mean callbacks should be the first thing initialized?

We also have correctness issues for these hooks when callbacks are passed directly to the Trainer vs configured from the LightningModule: https://github.com/PyTorchLightning/pytorch-lightning/blob/f407a00cec59886cbaf8816630f6760e34926bd5/pytorch_lightning/core/lightning.py#L1140-L1142

Therefore, as a user, it seems more concise to write this:

trainer = Trainer(...)
run_all_my_fancy_logic_now(trainer)

# use the trainer here

vs this:

class MyCallback(Callback):
    def on_init_start(self, trainer) -> None:
        ...
    
    def on_init_end(self, trainer) -> None:
        ...
 
cb = MyCallback()
trainer = Trainer(callbacks=[cb])

cc @awaelchli @ananthsub @tchaton @carmocca @Borda @ninginthecloud

[awaelchli]

I agree this part

We also have correctness issues for these hooks when callbacks are passed directly to the Trainer vs configured from the LightningModule:

is probably the strangest about these hooks. Currently on_init_start is used by none and on_init_end is used by three of our own callbacks in Lightning: EarlyStopping, ModelCheckpoint, Progress bar. The instructions in these hooks could probably also be move to another hook. None of them modify the trainer.

For context, the init hooks were introduced way back in #889

[Erotemic]

Is there a strategy for migrating away from these hooks? I've been using them to create a custom logger:

    def on_init_end(self, trainer: 'pl.Trainer') -> None:
        import pathlib
        self.log_dir = pathlib.Path(trainer.log_dir)
        self.log_fpath = self.log_dir / 'text_logs.log'
        self._log = _InstanceLogger.from_instance(trainer, self.log_fpath)
        self._log.info('on_init_end')
        self._log.info('sys.argv = {!r}'.format(sys.argv))
        trainer.text_logger = self
        if self.args is not None:
            self._log.info('args_dict = {}'.format(ub.repr2(self.args.__dict__, nl=1, sort=0)))

Am I SOL when 1.8 drops?

[carmocca]

Have you tried the setup hook?

[iamnmn9]

Hey!
Is there any solution for this?
AttributeError: 'EarlyStopping' object has no attribute 'on_init_end'
I am trying to implement early stopping.

[awaelchli]

@NamanPundir Can you share more from that error message please? Please note that on_init_start and on_init_end have been deprecated and then eventually removed. So there is no way to "fix" it. If you needed that hook, an alternative is the setup() hook. Another option is to downgrade to Lightning version <= 1.7.x. Hope this helps.