[Bugfix] Fix unstable silu_mul+nvfp4 quant fusion test (#24370)

Signed-off-by: elvischenv <219235043+elvischenv@users.noreply.github.com>

tests/compile/test_silu_mul_quant_fusion.py

 # SPDX-License-Identifier: Apache-2.0
 # SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+from typing import cast
+
 import pytest
 import torch
 
 import vllm.envs as envs
+from tests.kernels.quantization.nvfp4_utils import quant_nvfp4_tensor
 from vllm._custom_ops import cutlass_scaled_fp4_mm, scaled_fp4_quant
 # yapf conflicts with isort for this block
 # yapf: disable
@@ -64,24 +67,27 @@ class TestSiluMulFp8QuantModel(torch.nn.Module):
 
 class TestSiluMulNvfp4QuantModel(torch.nn.Module):
 
-    def __init__(self, hidden_size: int, **kwargs):
+    def __init__(self, hidden_size: int, x: torch.Tensor, **kwargs):
         super().__init__()
         self.silu_and_mul = SiluAndMul()
-        self.w = torch.randint(256, (hidden_size, hidden_size // 2),
-                               dtype=FP4_DTYPE)
-        self.wscale = torch.randn(hidden_size,
-                                  hidden_size // 16).to(dtype=FP8_DTYPE)
-        self.wscale2 = torch.rand(1, dtype=torch.float32)
-        self.scale = torch.rand(1, dtype=torch.float32)
+
+        # create nvfp4 weight
+        w = torch.rand((hidden_size, hidden_size))
+        self.w, self.w_block_scale, self.w_global_scale = quant_nvfp4_tensor(w)
+
+        # get global scale offline
+        _, _, self.y_global_scale = quant_nvfp4_tensor(self.silu_and_mul(x))
+
+        self.alpha = 1.0 / (self.w_global_scale * self.y_global_scale)
 
     def forward(self, x):
         y = self.silu_and_mul(x)
-        y_quant, y_block_scale = scaled_fp4_quant(y, 1 / self.scale)
+        y_quant, y_block_scale = scaled_fp4_quant(y, self.y_global_scale)
         out = cutlass_scaled_fp4_mm(a=y_quant,
                                     b=self.w,
                                     block_scale_a=y_block_scale,
-                                    block_scale_b=self.wscale,
-                                    alpha=self.scale * self.wscale2,
+                                    block_scale_b=self.w_block_scale,
+                                    alpha=self.alpha,
                                     out_dtype=y.dtype)
         return out
 
@@ -95,8 +101,9 @@ class TestSiluMulNvfp4QuantModel(torch.nn.Module):
 @pytest.mark.parametrize("num_tokens", [64])
 @pytest.mark.parametrize("hidden_size", [128])
 @pytest.mark.parametrize(
-    "model_class", [TestSiluMulFp8QuantModel, TestSiluMulNvfp4QuantModel]
-    if is_nvfp4_supported() else [TestSiluMulFp8QuantModel])
+    "model_class",
+    cast(list[type], [TestSiluMulFp8QuantModel, TestSiluMulNvfp4QuantModel]
+         if is_nvfp4_supported() else [TestSiluMulFp8QuantModel]))
 # cuda_force_torch used to test torch code path on platforms that
 # cutlass_fp8_supported() == True.
 @pytest.mark.parametrize("cuda_force_torch",
@@ -111,6 +118,8 @@ def test_fusion_silu_and_mul_quant(num_tokens, hidden_size, model_class,
     torch.set_default_device("cuda")
     torch.set_default_dtype(torch.float16)
 
+    x = torch.rand(num_tokens, hidden_size * 2)
+
     # Reshape pass is needed for the fusion pass to work
     config = VllmConfig()
     config.compilation_config = CompilationConfig(
@@ -119,10 +128,10 @@ def test_fusion_silu_and_mul_quant(num_tokens, hidden_size, model_class,
 
     backend = TestBackend(NoOpEliminationPass(config), fusion_pass)
     model = model_class(hidden_size=hidden_size,
-                        cuda_force_torch=cuda_force_torch)
+                        cuda_force_torch=cuda_force_torch,
+                        x=x)
 
     # First dimension dynamic
-    x = torch.rand(num_tokens, hidden_size * 2)
     torch._dynamo.mark_dynamic(x, 0)
 
     result = model(x)
@@ -131,10 +140,15 @@ def test_fusion_silu_and_mul_quant(num_tokens, hidden_size, model_class,
     result2 = model2(x)
 
     # Check that it gives the same answer
+    if model_class == TestSiluMulFp8QuantModel:
+        atol, rtol = 1e-3, 1e-3
+    elif model_class == TestSiluMulNvfp4QuantModel:
+        atol, rtol = 1e-1, 1e-1
+
     torch.testing.assert_close(result[0].to(dtype=torch.float16),
                                result2[0].to(dtype=torch.float16),
-                               atol=1e-3,
-                               rtol=1e-3)
+                               atol=atol,
+                               rtol=rtol)
 
     # In pre-nodes, quant op should be present and fused kernels should not
     backend.check_before_ops(model.ops_in_model_before())

tests/kernels/quantization/nvfp4_utils.py

@@ -2,6 +2,7 @@
 # SPDX-FileCopyrightText: Copyright contributors to the vLLM project
 import torch
 
+from vllm._custom_ops import scaled_fp4_quant
 from vllm.scalar_type import scalar_types
 
 FLOAT4_E2M1_MAX = scalar_types.float4_e2m1f.max()
@@ -65,3 +66,10 @@ def break_fp4_bytes(a, dtype):
 
     # Reshape to final form
     return values.reshape(m, n * 2).to(dtype=dtype)
+
+
+def quant_nvfp4_tensor(a: torch.Tensor):
+    a_global_scale = ((FLOAT8_E4M3_MAX * FLOAT4_E2M1_MAX) /
+                      torch.abs(a).max().to(torch.float32))
+    a_quant, a_block_scale = scaled_fp4_quant(a, a_global_scale)
+    return a_quant, a_block_scale, a_global_scale