Add predict_epoch_end and predict_step_end methods to LightningModule

[ConnectWithMehrshad]

🚀 Feature

Add predict_epoch_end and predict_step_end methods to LightningModule

Why aren't there these two methods?

Motivation

I think these methods are very useful. for example, putting the results of each batch together.

[ananthsub]

@mehrshadhosseini28 this is closely related to #8479

though for predict_epoch_end, you could locally keep track of the predict_step return values in your LightningModule and then post-processing them in on_predict_epoch_end

[stale]

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