Adding a new `align_to_words` param to qa pipeline.
Author: Narsil
Pull request: yes

# What does this PR do?

Adding a new `align_to_words` param to qa pipeline.

The problem lies that the "word" alignment really depends on the tokenizer
"pre_tokenizer" property, which might well not exist, especially on japanese
and other non space separated langages.

This new parameter allows to disable the "word" alignement for better/simpler
output for these kind of tokenizers.

Fixes #17706

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