Adding `top_k` argument to `text-classification` pipeline. #17606

- Deprecate `return_all_scores` as `top_k` is more uniform with other pipelines, and a superset of what `return_all_scores` can do. BC is maintained though. `return_all_scores=True` -> `top_k=None` `return_all_scores=False` -> `top_k=1` - Using `top_k` will imply sorting the results, but using no argument will keep the results unsorted for backward compatibility.