CCCL tuning_radix_sort.cuh teaches: when one kernel in a chain is unavailable,
replace ONLY that kernel while keeping downstream native ops alive.
Our MoE chain: topk_softmax → moe_align_block_size → invoke_fused_moe_kernel
BI-V100 ixformer lacks vllm_moe_topk_softmax, which killed the ENTIRE chain
and forced 100% PyTorch fallback (_pure_pytorch_experts: 256x F.linear loop).
Fix: Add try/except in topk_softmax with PyTorch fallback (softmax+topk).
Now the chain can proceed to native align+invoke kernels if they exist.
Also: dont permanently disable native path after first failure — retry once.
CCCL source: catch2_test_device_radix_sort_pairs.cu + tuning_radix_sort.cuh
Maps to: _custom_ops.py (topk_softmax) + qwen3_5.py (MoE forward)