Fix #16660 (tokenizers setters of ids of special tokens)
# What does this PR do?

Fixes #16660

@LysandreJik

It seems no one was using those setters because the code is quite old, but here's a fix if you want to merge it.
_The documentation is not available anymore as the PR was closed or merged._<|||||>Thank you very much for sharing your solution, it looks great! :hugs: 

Just to be able to merge this change on main I think it would be good to add a test to make sure that future changes don't break the behaviour you just fixed - I let @LysandreJik confirm.

This new test would surely have its place in `tests/test_tokenization_common.py`. If you ever need more help or don't have time to do it, don't hesitate to tell us!<|||||>Sure!

I've just added the checks into an existing test. Some things could be done differently depending on your preferences:
a) Simplify and assume that `len(tokenizer) > 0` always.
b) Add a dummy token before testing the setters when `len(tokenizer) == 0`. I've chosen not to do it because you may want to have tests as independent as possible, and this change would make the test fail when the method to add tokens fails.

Please let me know.<|||||>I ended up creating a different method for this test because I've seen at least two models where the ids shouldn't be set and I needed to disable only this test for those tokenizers.