MultiLabelClassificationTask predict step

[isaaccorley]

This PR is similar to #790 and adds a predict_step method to MultiLabelClassificationTask 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]

Not sure why codecov is complaining. When I go to the link it only shows coverage dips on scripts I didn't modify.