Suppress Warning in PredictionEpochLoop

[borchero]

What does this PR do?

This PR gets rid of a warning that is emitted every time on_run_start is run on the PredictionEpochLoop. At the time of the call to _get_batch_indices, the parameter return_predictions passed to on_run_start has not been set and the following warning is emitted in any case: "Lightning couldn't infer the indices fetched for your dataloader."

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