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/mla_preprocess/op_kernel/kernel/set_fpc.h

@@ -0,0 +1,38 @@
+/*  Adapted from
+ *      https://gitee.com/ascend/ascend-transformer-boost.git
+ *
+ * Copyright (c) 2024 Huawei Technologies Co., Ltd.
+ * This file is a part of the CANN Open Software.
+ * Licensed under CANN Open Software License Agreement Version 1.0 (the "License").
+ * Please refer to the License for details. You may not use this file except in compliance with the License.
+ * THIS SOFTWARE IS PROVIDED ON AN "AS IS" BASIS, WITHOUT WARRANTIES OF ANY KIND, EITHER EXPRESS OR IMPLIED,
+ * INCLUDING BUT NOT LIMITED TO NON-INFRINGEMENT, MERCHANTABILITY, OR FITNESS FOR A PARTICULAR PURPOSE.
+ * See LICENSE in the root of the software repository for the full text of the License.
+ */
+#ifndef INCLUDE_SET_FPC_H
+#define INCLUDE_SET_FPC_H
+
+#include "hardware.h"
+#include "kernel_tensor.h"
+
+/////////////////////////////////////////////////////
+// SetQuantPreAddr
+/////////////////////////////////////////////////////
+template <ArchType ArchTag, typename DataType>
+struct SetQuantPreAddr {
+    __aicore__ SetQuantPreAddr(AscendC::LocalTensor<DataType> quantPreTensor) {};
+};
+
+template <typename DataType>
+struct SetQuantPreAddr<ArchType::ASCEND_V220, DataType> {
+    static constexpr uint32_t QUANT_PRE_ADDR_MASK = 0xffff;
+    static constexpr uint32_t USELESS_BIT_NUM = 7;
+    static constexpr uint32_t QUANT_PRE_BIT_POS_IN_FPC = 8;
+
+    __aicore__ SetQuantPreAddr(AscendC::LocalTensor<DataType> quantPreTensor)
+    {
+        uint64_t quantPreAddr = (uint64_t)(__fbuf__ uint64_t *)quantPreTensor.GetPhyAddr();
+        AscendC::SetFixPipeConfigImpl(quantPreTensor);
+    };
+};
+#endif