Better heuristic for token-classification pipeline.
Author: Narsil
Pull request: yes

# What does this PR do?

Relooking at the problem makes thing actually much simpler,
when we look at ids from a tokenizer, we have no way in **general**
to recover if some substring is part of a word or not.

However, within the pipeline, with offsets we still have access to the
original string, so we can simply look if previous character (if it
exists) of a token, is actually a space. This will obviously be wrong
for tokenizers that contain spaces within tokens, tokenizers where
offsets include spaces too (Don't think there are a lot).

If will incorrectly fuse any punctuation too !  (" I am a robot!"). But that is already much better than what currently happens.

This heuristic hopefully is fully bc and still can handle non-word based
tokenizers.

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Fixes https://github.com/huggingface/transformers/issues/11887
Fixes https://github.com/huggingface/transformers/issues/12593


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