use combo kernel to fuse qk-norm and qk-rope (vllm-project#26682)

Signed-off-by: Boyuan Feng <boyuan@meta.com>
Signed-off-by: 0xrushi <6279035+0xrushi@users.noreply.github.com>

Author: BoyuanFeng

vllm/config/compilation.py

@@ -513,6 +513,16 @@ def __post_init__(self) -> None:
         if isinstance(self.pass_config, dict):
             self.pass_config = PassConfig(**self.pass_config)
 
+        if (
+            is_torch_equal_or_newer("2.9.0.dev")
+            and "combo_kernels" not in self.inductor_compile_config
+            and "benchmark_combo_kernel" not in self.inductor_compile_config
+        ):
+            # use horizontal fusion, which is useful for fusing qk-norm and
+            # qk-rope when query and key have different shapes.
+            self.inductor_compile_config["combo_kernels"] = True
+            self.inductor_compile_config["benchmark_combo_kernel"] = True
+
         # migrate the deprecated flags
         if not self.use_cudagraph:
             logger.warning(