Make the order of additional special tokens deterministic
In `SpecialTokensMixin.all_special_tokens_extended`, deduplication is performed by `all_toks = list(set(all_toks))`. However, this will change the ordering of additional special tokens, and the order depends on the hash seed of set data structure. This will result in non-deterministic id of additional special tokens added to `AutoTokenizer.from_pretrained` method. Therefore, I changed this problematic line to `all_toks = sorted(list(set(all_toks)))`.