Examples scripts using the TextPairClassificationProcessor are not working

[Timoeller]

Describe the bug When trying to test our TextPairClassificationProcessor I realized a lot of our example scripts are currently not working.

Example script text_pair_classification.py

• I think we have to upload the dataset "asnq_binary" for it to work. @brandenchan
• inheriting from TextclassificationProcessor might be failing: in the dataset_from_dicts method that we now call directly from the TextclassificationProcessor there we just convert "text" and not "text_b" as well @Timoeller

Example script passage_ranking.py

It throws an Exception("No task initialized....") @brandenchan

[Timoeller]

We should make Textpairclassification work to be able to support reranking in haystack like mentioned in https://github.com/deepset-ai/haystack/issues/423