[XPU] support token-slice for encoder

[TR666]

PR devices

XPU

PR types

New features

PR changes

PASS

Description

适配encoder的剪枝逻辑，支持multi-encoder的每层seq长度不一样
去掉mask结构中，多余的slice算子

[paddle-bot]

Thanks for your contribution!

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}