What does this PR do?

Fix hashing mechanism to be process independent. Typically hash doesn't generate the same hash when using different process. So this makes the maximum number of occurence of a text to be num_proc instead of 1 when deduplicating.

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• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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• Did you write any new necessary tests?

Who can review?

@lvwerra @loubnabnl