[CI] Shard Quantization job into 4 parallel shards
The Quantization test job runs ~71-75 min wall as a single job, over the 30-min
target. Split it with Buildkite parallelism + pytest-shard (mirroring the
existing 'Kernels Quantization Test %N' pattern): parallelism: 4, each shard
running a disjoint slice of tests/quantization via --shard-id/--num-shards.
Preserves device/key/env/torchao pin and the -k selection; per-shard timeout
lowered 75 -> 40 min.
