[gfx1250] fused_moe: require GUGU gu_interleave layout

[lalala-sh]

Summary

• gfx1250 grouped GEMM only consumes GUGU (gate/up row-interleaved) w1, not 16-row block shuffle.
• fused_moe now raises if G1U1 callers on gfx1250 pass a gate_mode other than interleave.

Test plan

• On gfx1250, run grouped GEMM UT with gate_mode=interleave (should still pass accuracy).
• On gfx1250, call fused_moe with G1U1 + gate_mode=separated and confirm it raises ValueError.
• Non-gfx1250 paths unchanged.

Made with Cursor

[github-actions]

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