add mla_preprocess kernel (#3226)

### What this PR does / why we need it?

- Adds the `mla_preprocess` custom kernel to provide an optimized
pre-processing operator for Multi-head Latent Attention (MLA) on Ascend
NPUs.
- Wires the new kernel into the C++ extension pipeline so vLLM can
invoke it directly, cutting Python-side tensor shuffling and memory
copies that previously bottlenecked MLA compilation paths.

### Does this PR introduce any user-facing change?

- No. The change only introduces a low-level kernel; public APIs and
inference behavior remain unchanged.

### How was this patch tested?

- Dedicated Ascend kernels are not covered by our CI yet, so no extra
automated tests were added. Future MLA-focused regression runs will
cover this path.

- vLLM version: v0.11.0

Signed-off-by: Chen Chen <0109chenchen@gmail.com>

csrc/torch_binding_meta.cpp

@@ -81,6 +81,41 @@ at::Tensor sgmv_expand_meta(at::Tensor &x, at::Tensor &weight, at::Tensor &lora_
     return y_out;
 }
 
+std::tuple<at::Tensor &, at::Tensor &, at::Tensor &, at::Tensor &> mla_preprocess(
+    const at::Tensor &hiddenState,
+    const at::Tensor &gamma0,
+    const at::Tensor &beta0,
+    const at::Tensor &wdqkv,
+    const at::Tensor &descale0,
+    const at::Tensor &gamma1,
+    const at::Tensor &beta1,
+    const at::Tensor &wuq,
+    const at::Tensor &descale1,
+    const at::Tensor &gamma2,
+    const at::Tensor &cos,
+    const at::Tensor &sin,
+    const at::Tensor &wuk,
+    const at::Tensor &kv_cache,
+    const at::Tensor &kv_cache_rope,
+    const at::Tensor &slotmapping,
+    const at::Tensor &quant_scale0,
+    const at::Tensor &quant_offset0,
+    const at::Tensor &bias0,
+    const at::Tensor &quant_scale1,
+    const at::Tensor &quant_offset1,
+    const at::Tensor &bias1,
+    const c10::optional<at::Tensor> &ctkv_scale,
+    const c10::optional<at::Tensor> &q_nope_scale,
+    c10::optional<c10::string_view> cache_mode,
+    c10::optional<c10::string_view> quant_mode,
+    at::Tensor &q_out0,
+    at::Tensor &kv_cache_out0,
+    at::Tensor &q_out1,
+    at::Tensor &kv_cache_out1)
+{
+    return {q_out0, kv_cache_out0, q_out1, kv_cache_out1};
+}
+
 
 } // namespace meta
 } // namespace vllm_ascend
@@ -97,6 +132,7 @@ namespace {
     ops.impl("bgmv_expand", &vllm_ascend::meta::bgmv_expand_meta);
     // Sgmv expand
     ops.impl("sgmv_expand", &vllm_ascend::meta::sgmv_expand_meta);
-
+    // MLA preprocess
+    ops.impl("mla_preprocess", &vllm_ascend::meta::mla_preprocess);
 }
 }