Patch token classification pipeline
This PR patches issues found with the `TokenClassificationPipeline` since the merge of https://github.com/huggingface/transformers/pull/5970, namely not being able to load a slow tokenizer in the pipeline.

It also sets the `ignore_subwords` to `False` by default, as this does not work with the slow tokenizers. No release have been done since the introduction of that argument, so it is not a breaking change.
Merging now to ensure the bug is patched in v3.5.0. Will address your comments in a future PR @Narsil.