Fix KeyError in convert_to_native_format for dict vocab (#44452)
When loading tokenizers like vesteinn/ScandiBERT whose tokenizer_config
specifies XLMRobertaTokenizer (model=Unigram) but whose tokenizer.json
contains a dict-type vocab, the expression vocab[0] raises KeyError
because dict keys are strings, not integers. Add an isinstance(vocab,
list) guard so the list-to-tuple conversion is only attempted on list
vocabs.