[tokenizer] use use_auth_token for config
This PR forwards `use_auth_token` to `AutoConfig.from_pretrained()` when `AutoTokenizer.from_pretrained(mname, use_auth_token=True)` is used. Otherwise `AutoConfig` fails to retrieve `config.json` with 404.

Model's `from_pretrained` does the right thing.

@sgugger, @LysandreJik 