fix: move results' keys to device (#19813)

[clumsy]

What does this PR do?

Move registered results' keys to the current device. Otherwise internal tensors like cumulated_batch_size will stay on cpu after they are moved there in previous call to teardown().

Removing standalone register_key() method because it's not used outside of this module and we either end up calling .to() twice or might need to require the potential users to call it themselves.

Fixes #19813

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📚 Documentation preview 📚: https://pytorch-lightning--19814.org.readthedocs.build/en/19814/

[clumsy]

Please check if you have a chance, @awaelchli