[XPU] Register fake meta kernel for fp4_gemm (#50946)

Signed-off-by: Chaojun Zhang <chaojun.zhang@intel.com>
Co-authored-by: Kunshang Ji <kunshang.ji@intel.com>

vllm/_xpu_ops.py

@@ -56,6 +56,24 @@ if hasattr(torch.ops._xpu_C, "fp8_gemm_w8a16"):
         return torch.empty((M, N), dtype=input.dtype, device=input.device)
 
 
+if hasattr(torch.ops._xpu_C, "fp4_gemm"):
+
+    @register_fake("_xpu_C::fp4_gemm")
+    def _fp4_gemm_fake(
+        input: torch.Tensor,
+        weight: torch.Tensor,
+        scale_act: torch.Tensor,
+        scale_wei: torch.Tensor,
+        out_dtype: torch.dtype | None = None,
+        bias: torch.Tensor | None = None,
+    ) -> torch.Tensor:
+        input_2d = input.view(-1, input.shape[-1])
+        M = input_2d.size(0)
+        N = weight.size(-1)
+        result_dtype = out_dtype if out_dtype is not None else torch.get_default_dtype()
+        return torch.empty((M, N), dtype=result_dtype, device=input.device)
+
+
 if hasattr(torch.ops._xpu_C, "int4_gemm_w4a8"):
 
     @register_fake("_xpu_C::int4_gemm_w4a8")