auto_tuner offers AOCL-DLP candidates it cannot run, and caches non-computing backends as fastest

[lwandrebeck]

Summary

get_algo_candidates() defaults to {aocl_dlp_blocked, onednn_blocked} regardless
of build configuration. Since auto_tuner is the default matmul path, the default
path cannot compute anything in a --no-aocldlp build: the tuner picks
aocl_dlp_blocked and every call fails with "requires AOCL-DLP".

An unavailable candidate is worse than merely useless here. The evaluate phase
keeps whichever candidate timed fastest, and "returned without computing" times
as near-zero — so a backend that cannot run wins the comparison and is cached as
the best algorithm for that shape.

Reproduce

--no-aocldlp build, any host:

benchdnn --op=matmul --lowoha=true --m=512 --k=512 --n=512 \
         --sdt=f32 --wdt=f32 --ddt=f32 --kernel_name=auto_tuner

Fails immediately. Setting ZENDNNL_MATMUL_AUTO_ALGO_CANDIDATES="2,10" works
around it, which confirms the candidate list is the only obstacle.

Suggested fix

Select the default from what the build actually contains. Two related points found
while fixing this:

1. The tuner should not time or cache a call that did not compute. The dispatch
   already signals this by rewriting kernel to the AOCL-DLP marker; there are
   three places that need the check, including the map seeding, where a ~0 ms seed
   makes an entry unbeatable for the process lifetime.
2. It should retry the next candidate rather than leave the output untouched.
   Without that, a candidate list spanning backends with different dtype coverage
   cannot work — the skip phase runs candidates[0] before any timing exists.

Also worth noting: the tuner bypasses should_use_mm_partitioner() entirely
(matmul_execute() handles auto_tuner and returns first), so a tuner-selected
LIBXSMM runs unpartitioned. On an A10-8770E that was 11.5 ms vs 6.9 ms for the
same shape requested by name — the tuner both measures and executes the slower
form.

Reference branch: https://github.com/lwandrebeck/BullDNN/tree/upstream/autotuner-aocl-candidates