feat: xllm MoE CUDA kernels — fused_topk + compute_index + combine

3 MoE kernel files adapted for corex:
  moe_fused_topk.cu: LOG(FATAL)→TORCH_CHECK, +torch/extension.h
  moe_compute_index.cu: CHECK_LE→TORCH_CHECK, uses cub::BlockScan (corex CUB)
  moe_combine.cu: fixed duplicate include, +torch/extension.h

New pybind binding: xllm_moe_bind.cpp
  → moe_fused_topk(gating, topk, renormalize, bias, scoring_func)
  → moe_compute_index(expert_id, num_experts)
  → moe_combine_result(gemm2, weights, N, topk)

AST verification added for all 3 functions

ex_engine/xllm_kernels/cuda/bindings/xllm_moe_bind.cpp

@@ -0,0 +1,34 @@
+// xllm_moe_bind.cpp — pybind11 for MoE CUDA kernels
+#include <torch/extension.h>
+#include <optional>
+#include <tuple>
+
+namespace xllm::kernel::cuda {
+std::tuple<torch::Tensor, torch::Tensor> moe_fused_topk(
+    torch::Tensor& gating_output, int64_t topk, bool renormalize,
+    const std::optional<torch::Tensor>& correction_bias,
+    const std::string& scoring_func);
+
+std::tuple<torch::Tensor, torch::Tensor, torch::Tensor> moe_compute_index(
+    const torch::Tensor& expert_id, int64_t num_experts);
+
+torch::Tensor moe_combine_result(
+    const torch::Tensor& gemm2, const torch::Tensor& reduce_weight,
+    int64_t N, int32_t topk);
+}
+
+PYBIND11_MODULE(TORCH_EXTENSION_NAME, m) {
+    m.def("moe_fused_topk", &xllm::kernel::cuda::moe_fused_topk,
+          "MoE fused topk (softmax or sigmoid routing)",
+          py::arg("gating_output"), py::arg("topk"),
+          py::arg("renormalize") = true,
+          py::arg("correction_bias") = py::none(),
+          py::arg("scoring_func") = "softmax");
+    m.def("moe_compute_index", &xllm::kernel::cuda::moe_compute_index,
+          "MoE compute permutation index (histogram + prefix_sum + place)",
+          py::arg("expert_id"), py::arg("num_experts"));
+    m.def("moe_combine_result", &xllm::kernel::cuda::moe_combine_result,
+          "MoE combine (reorder + weighted sum)",
+          py::arg("gemm2"), py::arg("reduce_weight"),
+          py::arg("N"), py::arg("topk"));
+}