Fix undefined TUNE_LOAD_ALGORITHM in scan benchmark .variant builds

[gonidelis]

cub/benchmarks/bench/scan/policy_selector.h consumes TUNE_LOAD_ALGORITHM/TUNE_LOAD_MODIFIER/TUNE_STORE_ALGORITHM, but sum.cu's %RANGE% axes only define TUNE_TRANSPOSE/TUNE_LOAD. The translation block every other lookback bench carries (partition, select, by_key) is missing from the scan chain.

That results for every scan.exclusive.sum variant build to fail with "identifier TUNE_LOAD_ALGORITHM is undefined". The scan tuning is broken upstream on every architecture. This is invisible in CI because .variant targets are never built there which is something that has led to other problems in the past.

I ran a dummy tuning to verify that the fix works since no tests exist.

(cccl) root ➜ /src/fixtest/cccl (scan_tuning_bug) $  CIQ_GPU=0 CIQ_POOL=32 CIQ_GENERATIONS=1 python3 /src/compileiq_search.py \
    -R '^cub\.bench\.scan\.exclusive\.sum$' -a 'T{ct}=I32' -a 'OffsetT{ct}=I32' \
    -DCMAKE_CUDA_ARCHITECTURES=native
cmake ../.. -DCUB_ENABLE_BENCHMARKS=YES -DCUB_ENABLE_TUNING=YES -DCCCL_ENABLE_THRUST=OFF -DCCCL_ENABLE_LIBCUDACXX=OFF -DCCCL_ENABLE_CUB=ON -DCCCL_ENABLE_TESTING=OFF -DCUB_ENABLE_DIALECT_CPP11=OFF -DCUB_ENABLE_DIALECT_CPP14=OFF -DCUB_ENABLE_DIALECT_CPP17=ON -DCUB_ENABLE_DIALECT_CPP20=OFF -DTHRUST_IGNORE_DEPRECATED_CPP_DIALECT=ON -DCUB_ENABLE_RDC_TESTS=OFF -DCMAKE_BUILD_TYPE=Release -DCMAKE_CUDA_ARCHITECTURES=native
 ctk:  13.4.46
cccl:  monorepo-5367-g0bfeaa0611
Driver-side probe evaluation: {'ipt': 7, 'tpb': 128, 'ns': 0, 'dcid': 0, 'l2w': 0, 'trp': 0, 'ld': 0}
cub.bench.scan.exclusive.sum.ipt_7.tpb_128.ns_0.dcid_0.l2w_0.trp_0.ld_0 0.4714180865641916
Probe score: 0.4714180865641916

ping @bernhardmgruber

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• cub/benchmarks/bench/scan/policy_selector.h

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📝 Walkthrough

Summary by CodeRabbit

• Performance Improvements
  • Improved scan performance tuning for configurations that do not use lookahead.
  • Memory loading and storing strategies are now selected more effectively across supported configurations.
  • No user-facing API or behavior changes are expected.

Walkthrough

Changes

Scan tuning

Layer / File(s)	Summary
Non-lookahead policy tuning cub/benchmarks/bench/scan/policy_selector.h	Defines algorithm and load-modifier macros from TUNE_TRANSPOSE and TUNE_LOAD for non-lookahead scan policy construction.

Suggested reviewers: bernhardmgruber

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