add predict_step to RegressionTask

[isaaccorley]

Related to #813. This PR adds a predict_step method to RegressionTask so that users can utilize PyTorch Lightning's trainer.predict() function which will automatically loop and predict over a PyTorch DataLoader e.g.:

preds = trainer.predict(model=task, dataloaders=datamodule.test_dataloader())

# Or if your datamodule has a `predict_dataloader` method defined
preds = trainer.predict(model=task, datamodule=datamodule)

[isaaccorley]

I also found the reason why we need the Tensor type hint is due to RegressionTask.forward (and other tasks) returning Any and not Tensor so mypy assumes that calling self(x) returns Any.