bug for custom classifier in adversarial training plan

[SabrinaRichter]

When trying to use a custom classifier in the adversarial training plan, i get an scvi 'AdversarialTrainingPlan' object has no attribute 'n_output_classifier' error, I think because it's not set here https://github.com/scverse/scvi-tools/blob/dcadecbc26979af916b92ba07c979535a4a32a48/src/scvi/train/_trainingplans.py#L526 similar to the case above, but it's used 6 lines further down.