[Perf] Eliminate padding and slicing op for GPT-OSS with Flashinfer MXFP4 MXFP8 MoE (#30647)

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

tests/compile/fusions_e2e/conftest.py

@@ -82,6 +82,10 @@ def run_e2e_fusion_test(monkeypatch, caplog_mp_spawn):
                 f"attention backend '{attn_backend.backend.name}'"
             )
 
+        # TODO: remove this after finishing migration from envs to model kwargs
+        if model_name == "openai/gpt-oss-20b":
+            monkeypatch.setenv("VLLM_USE_FLASHINFER_MOE_MXFP4_MXFP8", "1")
+
         # Disable, compile cache to make sure custom passes run.
         # Otherwise, we can't verify fusion happened through the logs.
         monkeypatch.setenv("VLLM_DISABLE_COMPILE_CACHE", "1")

tests/compile/fusions_e2e/models.py

@@ -162,3 +162,12 @@ deepseek_v3_fp8 = ModelFusionInfo(
         # async_tp=n_layers * 2,
     ),
 )
+
+gpt_oss_20b = ModelFusionInfo(
+    model_name="openai/gpt-oss-20b",
+    matches=lambda n_layers: Matches(
+        ar_rms_fusion=n_layers * 2 + 1,
+        sequence_parallel=n_layers * 2 + 1,
+        async_tp=n_layers * 2,
+    ),
+)

tests/compile/fusions_e2e/test_tp2_ar_rms.py

@@ -20,6 +20,7 @@ from .models import (
     FLASHINFER_MLA_ATTN,
     TRITON_ATTN,
     deepseek_v3_fp8,
+    gpt_oss_20b,
     llama3_8b,
     llama3_8b_fp4,
     llama3_8b_fp8,
@@ -158,7 +159,7 @@ def test_tp2_ar_rms_fp4_fusions(
 @multi_gpu_test(num_gpus=2)
 @pytest.mark.parametrize(
     "model_name, matches_fn, model_kwargs, hf_overrides",
-    [llama3_8b, qwen3_a3b],
+    [llama3_8b, qwen3_a3b, gpt_oss_20b],
 )
 @pytest.mark.parametrize("attn_backend", [TRITON_ATTN])
 @pytest.mark.parametrize("n_layers", [4])