Sort out the dataloader idx logic for evaluation

[carmocca]

What does this PR do?

• Save the current dataloader idx in the ResultCollection instead of the LightningModule
• Avoid leaking the dataloader idx logic from the EvaluationLoop (where it should be contained) to the EvaluationEpochLoop

Follow-up to #10810

This will enable an optimization where we don't compute the results multiple times when multiple dataloaders are used during evaluation.

Does your PR introduce any breaking changes? If yes, please list them.

None

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cc @Borda @justusschock @awaelchli @akihironitta @carmocca @edward-io @ananthsub @ninginthecloud

[tchaton]

Very neat ! A very good step toward separating the LightningModule.