[Kernel][Bugfix] Fixup some warnings in nvfp4_blockwise_moe when CUDA < 12.8 (#20324)

Signed-off-by: Tyler Michael Smith <tyler@neuralmagic.com>

csrc/quantization/fp4/nvfp4_blockwise_moe_kernel.cu

@@ -335,8 +335,10 @@ void run_fp4_blockwise_scaled_group_mm(
   TORCH_CHECK(status == cutlass::Status::kSuccess, "Failed to run GEMM");
 }
 
+#if defined ENABLE_NVFP4 && ENABLE_NVFP4
 constexpr auto FLOAT4_E2M1X2 = at::ScalarType::Byte;
 constexpr auto SF_DTYPE = at::ScalarType::Float8_e4m3fn;
+#endif
 
 #define CHECK_TYPE(x, st, m) \
   TORCH_CHECK(x.scalar_type() == st, ": Inconsistency of Tensor type:", m)