llama : fix KV shift for qwen2vl

[ngxson]

Fix #13865

Provided a model with 3 mrope sections, a rotated vector looks like this: [time, time+x, time+y]

This works with the assumption that mrope with x == y == 0 is identical to doing a neox rope with theta = +time

For example, when we want to shift the time to time-1, that mean we shift from [time, time+x, time+y] to [time-1, time-1+x, time-1+y] ; We can simply apply neox rope with theta = -1 to archive the same effect

[ggerganov]

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