Inconsistent API for on_predict_epoch_end

[ananthsub]

🚀 Feature

Background
We are auditing the Lightning components and APIs to assess opportunities for improvements:

• Review Lightning architecture & API #7740
• https://docs.google.com/document/d/1xHU7-iQSpp9KJTjI3As2EM0mfNHHr37WZYpDpwLkivA/edit#

One item that came up was on_predict_epoch_end() defined on the ModelHooks Mixin. This accepts an outputs: List[Any] argument. However, this is inconsistent with the other model hooks of the same type: on_train_epoch_end, on_validation_epoch_end, and on_test_epoch_end

Motivation

API consistency with other epoch end model hooks.

Pitch

• Add a prediction_epoch_end hook to the LightningModule
• Deprecate the outputs argument from on_predict_epoch_end in v1.5 and remove entirely in v1.7
• Update the prediction loop to only cache predictions if prediction_epoch_end is implemented

Users can optionally avoid this entirely and cache their prediction outputs as needed by implementing this logic in predict_step directly

Alternatives

Keep as is?

Additional context

cc @Borda @tchaton @justusschock @awaelchli @carmocca @ninginthecloud @daniellepintz @rohitgr7

[ishtos]

I'd like to work on this, could you assign this issue to me?