Re-enable LightningCLI test disabled due to bug in jsonargparse

[mauvilsa]

What does this PR do?

Re-enables the test_ddpstrategy_instantiation_and_find_unused_parameters test which was disabled because of omni-us/jsonargparse#473

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📚 Documentation preview 📚: https://pytorch-lightning--19683.org.readthedocs.build/en/19683/

[awaelchli]

Thanks @mauvilsa, very nice!