get_spans_from_bio: Start new span for previous S- if class also changed

[Lingepumpe]

In previous flair versions (e.g. 0.10) get_span_from_bio would start a new span if the previous tag was a "S-" token of a different class than the current token, see:
https://github.com/flairNLP/flair/blob/v0.10/flair/data.py#L698

It happens that our production model semi-frequently produces these kinds of (invalid BIOES) prediction, and that the new span extraction performs worse on our data.
This adds back this special check for previous tag "S-", making span calculation more similar to what it was in 0.10, and remaining the same for all 100% valid BIOES tagging.

Also included are some minor code tweaks to the function to make it prettier.

[alanakbik]

Hello @Lingepumpe thanks for fixing this.

Reviewing the code I think the line

        if bioes_tag[0:2] == "S-" and previous_tag[2:] != bioes_tag[2:]:
            starts_new_span = True

can be removed altogether, since before this, we already check for S- alone:

        # begin and single tags start new spans
        if bioes_tag[0:2] in {"B-", "S-"}:
            starts_new_span = True

[Lingepumpe]

Removed the unnecessary if, also wrote the whole "starts_new_span = True" conditions a bit more compactly

[alanakbik]

Thanks @Lingepumpe, looks great!