feat: add new callback hook on_before_optimizer_setup

[deependujha]

What does this PR do?

Partially addresses #19658

This PR introduces a new callback hook on_before_optimizer_setup to provide a safe point between configure_model() and configure_optimizers().

It’s primarily designed to fix incompatibility between BaseFinetuning and LightningModules that define submodules inside configure_model (e.g., FSDP, DeepSpeed, etc.).

---

⚙️ Hook order (new)

setup
→ configure_model
→ on_before_optimizer_setup  ← NEW
→ configure_optimizers
→ on_fit_start

---

Example

class MyFinetuningCallback(Callback):
    def on_before_configure_optimizers(self, trainer, pl_module):
        # freeze parameters before optimizer creation
        for param in pl_module.backbone.parameters():
            param.requires_grad = False

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Use case

BaseFinetuning’s freeze_before_training() must execute before optimizers are created.
Previously, there was no hook between configure_model() and configure_optimizers(), making it incompatible with modules that instantiate submodules in configure_model().

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📚 Documentation preview 📚: https://pytorch-lightning--21272.org.readthedocs.build/en/21272/

[Copilot]

Pull Request Overview

This PR introduces a new callback hook on_before_optimizer_setup to provide a safe point between configure_model() and configure_optimizers(). The hook is designed to enable callbacks like BaseFinetuning to safely modify model parameters (such as freezing) before optimizers are created, particularly for LightningModules that instantiate submodules within configure_model().

• Adds the new on_before_optimizer_setup hook to both callback and module hook interfaces
• Updates the trainer execution flow to call this hook only during fitting stage
• Provides comprehensive test coverage for the new hook timing and behavior

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/lightning/pytorch/trainer/trainer.py	Added hook invocation in trainer's _run method during fitting stage
src/lightning/pytorch/core/hooks.py	Defined the on_before_optimizer_setup hook for LightningModule
src/lightning/pytorch/callbacks/callback.py	Defined the on_before_optimizer_setup hook for Callback interface
tests/tests_pytorch/callbacks/test_callback_hooks.py	Added comprehensive test for hook timing and behavior
docs/source-pytorch/common/hooks.rst	Updated documentation to include the new hook in the execution order

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[GdoongMathew]

Lgtm!