[NVIDIA] Fix wrong symmetric sizes for fp4 cases (#12640)

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

@@ -1602,8 +1602,13 @@ class ModelOptNvFp4FusedMoEMethod(FusedMoEMethodBase):
                 x_sf = nvfp4_block_scale_interleave(x_sf)
 
             with use_symmetric_memory(get_tp_group()) as sm:
+                # The x might be packed in the case of fp4. So, use the output dim of the
+                # weight of the second GEMM.
                 symm_output = torch.empty(
-                    x.shape[0], x.shape[1], dtype=output_dtype, device=x.device
+                    x.shape[0],
+                    layer.w2_weight.shape[1],
+                    dtype=output_dtype,
+                    device=x.device,
                 )
                 sm.tag(symm_output)