[PretrainedTokenizer] is  <unk> a special token?
This confused me a lot as I was trying to add common tests for Marian.


I believe special_tokens_mask [must](
https://github.com/huggingface/transformers/blob/master/tests/test_tokenization_common.py#L445) return 0 for positions with unk_token_id, but `<unk>` is in `SpecialTokensMixin.all_special_ids`.

Relatedly, this [RobertaTokenizer](https://github.com/huggingface/transformers/blob/master/src/transformers/tokenization_roberta.py#L204) code suggests there should be a relationship between `all_special_ids` and `special_token_mask` logic.

Maybe `<unk>` shouldnt be in `SpecialTokensMixin.all_special_ids`?


@mfuntowicz @LysandreJik might understand the best!<|||||>This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.
