[Cherry pick to release/v0.5.15] Fix NVFP4 online quantization (#30397)

Co-authored-by: Brayden Zhong <brayden@radixark.ai>

python/sglang/srt/layers/quantization/nvfp4_online.py

@@ -85,6 +85,7 @@ class NvFp4OnlineConfig(ModelOptQuantConfig):
         self.use_per_token_activation = True
         self.is_checkpoint_fp8_serialized = is_checkpoint_fp8_serialized
         self.is_fp4_experts = False
+        self.dequant_fp4_to_fp8 = False
         self.activation_scheme = activation_scheme
         self.weight_block_size = weight_block_size
         self.use_mxfp8 = use_mxfp8