[BugFix] fix VL fp8 bug when moe token_num is 0

[ming1753]

Motivation

多模态模型可能存在仅有文本无图片，或者仅有图片无文本的情况，此时图像Moe或文本Moe会收到一个token_num=0的输入。Triton算子不支持。

Modifications

一种方法是拼接一个长度为1的token vector，不跳过kernel执行，并且在计算完成后切回真实长度。优点是这样理论上Prefill可以跑Cuda Graph，但是缺点是会造成轻微显存增长。
考虑到短期内prefill不需要跑Cuda Graph，现采用token_num=0时跳过执行的修复方式。

Usage or Command

无

Accuracy Tests

无

Checklist

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[paddle-bot]

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