Quantus.nlp support for custom models

[aaarrti]

Description of the problem

• (https://github.com/understandable-machine-intelligence-lab/Quantus/pull/224) is designed with a goal to support models from Hugging Face Hub 🤗 because it is currently the most popular model sharing service. Ideally, user should also be able to provide his model, implementing TextClassifier, this is however not tested.

Description of a solution

• Ensure Quantus compatibility with non HF models.
• Should be done after https://github.com/understandable-machine-intelligence-lab/Quantus/pull/224 is closed.