[Feature]: Refactor `aiter_shared_expert_fusion`

[robertgshaw2-redhat]

🚀 The feature, motivation and pitch

We have a special case in FusedMoE layer for aiter_shared_expert_fusion which creates various if branches spattered across the layer

We should factor this out

Alternatives

No response

Additional context

No response

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[github-actions]

CC @hongxiayang @tjtanaa @vllmellm for ROCm-related issue

[tjtanaa]

@robertgshaw2-redhat 👍 we will take a look.

[tjtanaa]

It seems this feature is coming to CUDA as well through this PR #28540 . We are working on a more generic abstraction.