llama-graph : fix text position for mrope

[ngxson]

Cont #13138

I misunderstood the original code:

        // TODO: add mrope pos ids somewhere else
        pos.resize(batch.n_tokens * 4);
        std::fill(pos.begin(), pos.end(), 0);
        for (int j = 0; j < batch.n_tokens * 3; j ++) {
            pos[j] = *st_pos_id + (j % batch.n_tokens);
        }
        batch.pos = pos.data();

Which means 3 first dims are the same, and 4th dim is 0, something like this:

1234...1234...1234...0000...

Thanks @mattjcly for spotting this!

[mattjcly]

Can confirm this fixes bugs we observed! Thanks @ngxson :)