[CPU][Zen] Route BF16 MoE inference through zentorch on AMD

[Priyjain-amd]

Routes CPU MoE on AMD Zen through a zentorch-backed fused MoE path in CPUFusedMOE, ahead of the existing AMX grouped-GEMM / OneDNN / per-expert PyTorch fallbacks:

• forward_zentorch — MoE FFN via torch.ops.zentorch.zentorch_fused_moe (standard [E, ...] expert weights; no cpu_prepack_moe_weight).
• is_zentorch_moe_supported() — capability check at CPUFusedMOE init: op registered, moe_config.is_act_and_mul when present, and activation in {silu, gelu, gelu_tanh, swigluoai}.

Dispatch gates on current_platform.is_zen_cpu() plus has_zentorch_op(["zentorch_fused_moe"]). When supported, __init__ sets forward_method = forward_zentorch and isa = "none" and returns early (no weight prepack). On Zen CPU when zentorch is unavailable or the layer is unsupported, init logs once and falls through to forward_grouped_gemm / forward_torch as today. No behaviour change on non-Zen CPUs, GPUs, or XPU.

Also adds _gelu_tanh_and_mul to _CPU_MOE_ACT_FN so the native CPU MoE path supports gelu_tanh when the zentorch fast path is not taken (e.g. Gemma 4).

[github-actions]

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[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @Priyjain-amd.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

[AndreasKaratzas]

LGTM
cc @tjtanaa could you review this one as well?

[mergify]

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Hi @Priyjain-amd, the pre-commit checks have failed. Please run:

uv pip install pre-commit>=4.5.1
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[github-actions]

✅ Triggered Buildkite CI #82700 for commit d59f161b1ab0.

[AndreasKaratzas]

/ci run

[mergify]

Hi @Priyjain-amd, the pre-commit checks have failed. Please run:

uv pip install pre-commit>=4.5.1
pre-commit install
pre-commit run --all-files

Then, commit the changes and push to your branch.

For future commits, pre-commit will run automatically on changed files before each commit.

[AndreasKaratzas]

/ci run

[github-actions]

✅ Triggered Buildkite CI #82796 for commit 783ffe4ee784.

[AndreasKaratzas]

/ci retry

[github-actions]

✅ Queued 4 failed job(s) for retry in Buildkite CI #82796.

[AndreasKaratzas]

/ci run

[github-actions]

✅ Triggered Buildkite CI #83170 for commit 74a02bf788f2.

[AndreasKaratzas]

/ci run

[github-actions]

✅ Triggered Buildkite CI #83176 for commit 290a99bb27ab.

[AndreasKaratzas]

/ci run

[github-actions]

✅ Triggered Buildkite CI #83204 for commit 3c233a0a0db5.