Fix StochasticWeightAveraging with infinite epochs

[SkafteNicki]

What does this PR do?

Fixes #21347
Fix the issue of using StochasticWeightAveraging with max_epochs=-1 which currently does not work.
Need custom logic to make sure the weight averaging does not stop training early.

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📚 Documentation preview 📚: https://pytorch-lightning--21396.org.readthedocs.build/en/21396/