[CI] Shard Quantization job into 4 parallel shards (≤30 min target) (#52328)

.buildkite/test_areas/quantization.yaml

@@ -2,10 +2,11 @@ group: Quantization
 depends_on:
   - image-build
 steps:
-- label: Quantization
+- label: Quantization %N
   device: h200_35gb
   key: quantization
-  timeout_in_minutes: 75
+  timeout_in_minutes: 40
+  parallelism: 4
   env:
     VLLM_USE_V2_MODEL_RUNNER: "0"
   source_file_dependencies:
@@ -18,7 +19,7 @@ steps:
   - uv pip install --system conch-triton-kernels
   # The SM90-only checkpoint currently contains a removed weight_chan_scale
   # parameter. It was not exercised by the previous L4 job.
-  - VLLM_TEST_FORCE_LOAD_FORMAT=auto pytest -v -s quantization/ --ignore quantization/test_blackwell_moe.py -k 'not test_compressed_tensors_w4a8_fp8'
+  - VLLM_TEST_FORCE_LOAD_FORMAT=auto pytest -v -s quantization/ --ignore quantization/test_blackwell_moe.py -k 'not test_compressed_tensors_w4a8_fp8' --shard-id=$$BUILDKITE_PARALLEL_JOB --num-shards=$$BUILDKITE_PARALLEL_JOB_COUNT
 
 - label: Quantized Fusions
   device: h200_35gb