Fix restoring trainer after `lr_find()`

[Callidior]

What does this PR do?

Fixes #12670: After using the LR finder and then training the model with the same trainer instance (which is the standard use-case), an incorrect learning rate schedule is used, leading to immediate divergence.

This bug was introduced in Lightning version 1.6 by commit a6a28e08d22f59f2468ff1049c67507df7519e7c, which removed two lines responsible for resetting the init_optimizers method of the trainer after it has been replaced with a wrapper changing the LR scheduler to one trying different learning rates.

This PR reintroduces these two lines and updates them to match the current codebase, where trainer.init_optimizers has been renamed to trainer.strategy.setup_optimizers.

Does your PR introduce any breaking changes? If yes, please list them.

No

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