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
a50adefdfc