CustomOp: Unify aiter impl into GroupedTopk

[xinyu-intel]

Purpose

a follow-up of #29575 (comment)

Test Plan

Test Result

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[chatgpt-codex-connector]

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[gemini-code-assist]

Code Review

This pull request refactors the ROCm AITer implementation for GroupedTopk by encapsulating the platform-specific logic within the GroupedTopk custom operator. This is achieved by introducing a forward_hip method in GroupedTopk and moving the AITer-related checks and function calls there. Consequently, the FusedMoE.select_experts method is simplified, removing conditional logic and direct calls to rocm_aiter_grouped_topk. The changes improve code structure and encapsulation, making the FusedMoE layer more platform-agnostic. The implementation appears correct and is a good improvement.

[xinyu-intel]

@MengqingCao please take a review, thx.

[zejunchen-zejun]

LGTM
Hi @tjtanaavllm Could you help this PR? Thank you

[MengqingCao]

LGTM, thx!