[Bugfix] Fix build issue around cutlass_fp4_group_mm on platforms without nvfp4

[StrongerXi]

Purpose

Fixes the undefined symbol issue (#23925 (comment)) by keeping the function definition of cutlass_fp4_group_mm even on non-nvfp4-supported platforms in csrc/quantization/fp4/nvfp4_blockwise_moe_entry.cu.

Similar to #23727.

Test Plan

Existing tests and build pass on platform without nvfp4 support (H100 in my case).

Test Result

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[gemini-code-assist]

Code Review

This pull request aims to fix a build issue for cutlass_fp4_group_mm on platforms without nvfp4 support by introducing a dispatch file. The approach is correct, but there is a critical error in the implementation of the new dispatch function that will cause a compilation failure. I've provided a fix for this issue.

[StrongerXi]

This file isn't always included in the build, I think that's why I ran into the undefined symbol error

vllm/CMakeLists.txt

Lines 600 to 606 in 1726e93

	if(${CMAKE_CUDA_COMPILER_VERSION} VERSION_GREATER_EQUAL 12.8 AND FP4_ARCHS)
	set(SRCS
	"csrc/quantization/fp4/nvfp4_quant_kernels.cu"
	"csrc/quantization/fp4/activation_nvfp4_quant_fusion_kernels.cu"
	"csrc/quantization/fp4/nvfp4_experts_quant.cu"
	"csrc/quantization/fp4/nvfp4_scaled_mm_kernels.cu"
	"csrc/quantization/fp4/nvfp4_blockwise_moe_kernel.cu")

[ProExpertProg]

This should be covered by #26077