fix: Make `convert_token_to_string` pickleable

[saattrupdan]

This moves the convert_token_to_string helper function out as an independent function, to make it pickleable.

This is causing issues when storing logits processors in a class that needs to be pickleable.

[saattrupdan]

@rlouf, does this seem good to you?

[rlouf]

Tests were failing for unrelated reasons, I just rebased on main

[RobinPicard]

Hi @saattrupdan ! We've recently released Outlines v1 that modifies a bit our models. What you were working on is now called VLLMOffline and is located in the outlines/models/vllm_offline.py file. Otherwise, it looks like what you propose is still applicable though.