### 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>
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