feat: introduce predict_epoch_end hook to LightningModule for being c…

[ishtos]

What does this PR do?

Fixes #8479

• outputs argument from on_predict_epoch_end is deprecated here

• introduce predict_epoch_end method to LightningModule here

• The prediction loop to cache prediction if prediction_epoch_end is implemented. To realize that, should_store_predictions property is deprecated and will be taken the place by _should_track_batch_outputs_for_epoch_end

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

None.

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[ishtos]

Might BasePredictionWriter be deprecated as well when prediction loop will cache outputs depends on whether predict_epoch_end is implemented or not?

[ishtos]

Should outputs be deprecated in v1.6 and removed in v1.8?

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[stale]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed in 7 days if no further activity occurs. If you need further help see our docs: https://pytorch-lightning.readthedocs.io/en/latest/generated/CONTRIBUTING.html#pull-request or ask the assistance of a core contributor here or on Slack. Thank you for your contributions.

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