[RFC] Deprecate `on_before_accelerator_backend_setup` hook

[daniellepintz]

Proposed refactor

Deprecate on_before_accelerator_backend_setup hook in v1.6 for removal in v1.8. Redirect users to the setup hook.
https://github.com/PyTorchLightning/pytorch-lightning/blob/5ad5ba54c0c477546d21daf75ac7b4748d7963a7/pytorch_lightning/callbacks/base.py#L63-L65

Motivation

I only see two external uses of this hook and 3 internal uses in https://www.grep.app/search?q=on_before_accelerator_backend_setup
Users can implement the setup hook instead.

Additional context

This hook was originally added in #5377
Part of #7740

cc @justusschock @awaelchli @akihironitta @rohitgr7 @tchaton @carmocca @Borda @ninginthecloud @daniellepintz

[ananthsub]

+1 - this hook is also confusing following the strategy refactor, since the Accelerator no longer directly controls the backend setup

[tchaton]

This hook was introduced to enable freezing model weights prior to creating the optimizers.

Now that accelerator has been refactored, I believe setup would be a better replacement and we could depreciate this and move the base fine-tuning callback to rely on setup.

[krishnakalyan3]

I would like to work on this!.

[krishnakalyan3]

@daniellepintz can you please point me to examples/steps on how to achieve this?.

[carmocca]

I agree with deprecating this, the hook is really specific. Using it doesn't paint a good picture design-wise so I think it'll be good to clean up, now that it's not necessary.

[daniellepintz]

Thanks @krishnakalyan3! Please see #10940 as an example. Lmk if you have any questions!

[awaelchli]

Strictly speaking, setup() is not a direct replacement/alternative for this deprecated hook.
the sequence of hook calls is this:

        self._data_connector.prepare_data()
        self._call_callback_hooks("on_before_accelerator_backend_setup")
        self.strategy.setup_environment()
        self._call_setup_hook()

Just saying.
But nobody uses this hook specifically to run right before setup_environment.