feat: implement 5 missing MoE ops — topk_softmax + token_index + expand + group_gemm + combine
Symbol dump from real device confirms: libixformer.so has 0 MoE symbols.
topk_softmax, moe_compute_token_index_api, moe_expand_input,
moe_w16a16_group_gemm, moe_output_reduce_sum — all missing.
Non-MoE symbols (silu_and_mul, rms_norm, flash_attn, reshape_and_cache,
rotary_embedding) are present and working.
Implementation strategy — use available primitives:
- topk_softmax: pure CUDA kernel (64-expert, shared-mem argmax)
- moe_compute_token_index: histogram + prefix_sum + scatter (3 kernels)
- moe_expand_input: gather kernel
- moe_w16a16_group_gemm: per-expert loop calling cuinferCustomGemm
(confirmed in libcuinfer.so symbol dump: cuinferCustomGemm exists)
- moe_output_reduce_sum: weighted combine kernel
All in ixformer::infer namespace so ix_full_bridge_v2.cpp links directly.
Compile: nvcc moe_ops_impl.cu + ix_full_bridge_v2.cpp → single .so
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