refactor: refactor trtllm-gen attention kernel integration code (#1289)

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## 📌 Description

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needed. -->
Simplify and unify the interface for trtllm-gen decode/prefill/mla
kernels, and add support for shared-kv (in MLA,
#1273).

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Co-authored-by: siyuanf <[email protected]>
Co-authored-by: Avery Yingyi Huang <[email protected]>
Co-authored-by: Zihao <[email protected]>

Author: 3 people