Fix AttributeError when using CombinedLoader in prediction

[awaelchli]

What does this PR do?

Fixes #11062

This is a partial fix to enable prediction with CombinedLoader without tracked sample indices until we figure out how we support batch_sampler with CombinedLoader.

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Did you have fun?

I made sure I had fun coding 🙃

Part of #1 (it's a lie, this is just here to avoid noisy GitHub bot)

cc @Borda @justusschock @awaelchli @ninginthecloud