result type of `argsort`: make it I64?

At present, result type of `argsort` is tensor of `I32`. It is better to be `I64`.

https://github.com/ggml-org/llama.cpp/blob/b8e09f08b9a91c0401bc67d17a17c90756420346/ggml/src/ggml-cpu/ops.cpp#L8648-L8650

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I am trying to use `ggml_set_rows`, and finding that it uses indices of `I64`. This is the motivation of creating this issue.