Use word_ids to get labels in run_ner
# What does this PR do?

As #8958 pointed out, the current way labels are computed in the `run_ner` script using offset mappings does not work for sentencepiece-based tokenizers. This PR fixes that using the `.word_ids` method which is more elegant and more reliable.

In passing it adds an early check that the tokenzier is fast (otherwise the script just doesn't work).

Fixes #8958
