[CI] Shard Kernels Quantization Test to 6 shards, pin AMD mirror at 2 (<20 min)

[khluu]

What

kernels-quantization-test runs 2 NVIDIA shards at ~40–45 min each (~85 min logical) — a Phase-2 long pole. Raise NVIDIA parallelism 2 → 6 to land each shard ~14 min (<20 min target), keeping the AMD mirror at its current 2 shards.

How

• NVIDIA parallelism: 2 → 6 (the pytest -v -s kernels/quantization --shard-id/--num-shards command already shards). Per-shard timeout_in_minutes 60 → 30. Coverage/selection unchanged.
• Pin mirror.amd.parallelism: 2 so the higher NVIDIA count does not multiply the AMD mirror. Depends on the ci-infra AMD-mirror parallelism override — Honor mirror.amd.parallelism so NVIDIA sharding cannot multiply AMD mirrors ci-infra#473 (amd-mirror-parallelism-override); without it the field is ignored (AMD would inherit 6), with it AMD stays 2. AMD command/coverage unchanged.

Sizing

~85 min logical ÷ 6 ≈ 14 min/shard; headroom under 20 min for nodeid-hash imbalance. Provisional — confirm/trim from the targeted run.

Validation — build #83916 (targeted kernels-quantization-test on #473, 6/6 green)

• Per-shard NVIDIA wall (min): 10.27 / 11.72 / 11.85 / 10.47 / 16.15 / 13.55 — max 16.15, all <20.
• Exact node union: per-shard "Running N" = 1032/992/1027/1004/1002/1026, Σ = 6083 = collected 6083 (disjoint partition).
• Fixed setup: 4.30–4.36 min/shard.
• Total GPU-min: 74.0 vs the 2-shard baseline 85 (−11) (within run variance; added per-shard setup offset by lower test time this run); wall dropped ~48→16.15 min (~3×).
• AMD2 selector-off render (real indexSelectLargeIndex: block: [308,0,0], thread: [95,0,0] Assertion srcIndex < srcSelectDimSize failed. #473 generator, head e006ba64): NVIDIA renders 6, AMD mirror pinned to exactly 2 (un-pinned it would inherit 6); AMD command byte-identical.

Part of the Phase-2 CI-overhaul sharding pass (kernels/basic-models workstream), one PR per key.

[khluu]

Superseded by consolidated kernels PR #54754 on current main.