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 a6a28e0, 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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Did you have fun?

Make sure you had fun coding 🙃

[carmocca]

Great!

[awaelchli]

FYI, this PR created an issue with convergence and I am investigating now what the cause is. Repro here: #14824