[XPU] refactor of block_attn param 'pos_emb_type'

[cqulilujia]

Motivation

重构XPU block_attention算子中的pos_emb_type参数，将pos_emb_type的赋值从框架下沉到自定义算子，增加算子的通用性，减少新模型的适配成本和出错风险。

[paddle-bot]

Thanks for your contribution!

[hong19860320]

LGTM

[codecov-commenter]

Codecov Report

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Additional details and impacted files

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[ddchenhao66]

LGTM

[iosmers]

LGTM