Deprecate LightningModule's write_predictions in favor of predict_step

[ananthsub]

🚀 Feature

Not a feature but a request for eventual removal:
The LightningModule has write_predictions and write_prediction_dict as an API. The LightningModule also now has predict_step, there's Trainer.predict(), and users can configure callbacks for this loop based on the predict hooks.

This means we can replace write_predictions with predict_step + a callback to output results to any destination (not just a file), which is more flexible than write_predictions

Motivation

Avoid confusion/redundancy for users as to which hook they should implement

write_predictions also reaches deep into the trainer internals which is bad practice. this also implies that the trainer is available, which is not strictly necessary for pure inference: as a user I should be able to generate batches on my own and run the forward pass without requiring access to the trainer or its internal evaluation loop

Pitch

Mark these APIs on the Lightning Module as deprecated in v1.3 and slated fo removal in v1.5. Replace write_predictions with predict_step + a callback to output results to any destination (not just a file), which is more flexible than write_predictions

Alternatives

Keep as is

Additional context