[Feature] support Assign token to update the content of a token

[ArthurZucker]

Very draft for now:

• handle potential collisions

fixes #1473

[HuggingFaceDocBuilderDev]

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[pritam-dey3]

We really need this feature! Is it tracked anywhere else?

[ArthurZucker]

oups sorry, yeah here is the good place will come back in a bit!

[ArthurZucker]

Okay, so this PR works for assigning, but the issue is that for Unigram based models if the world is not removed from the internal lattice, it can still be "encoded". Might be an issue.
We are also updating the added_tokens layer on top of vocab

[ArthurZucker]

Related to #1437 that does not work for unigram as if the token is in the vocabulary, even if it is special, it's still added to the unigram algorithm!

[jp1924]

hello @ArthurZucker
Is this work simply delayed, or has it been stopped due to a critical issue?

related issue: #1473, #31475

[ArthurZucker]

there is kind of a critical issue which is that a general solution is hard to find given how unigram works. Once backtracking BPE is merged i'll have another look