[CI] Shard Hybrid SSM NixlConnector PD accuracy tests into 3 config groups

Split hybrid-ssm-nixlconnector-pd-accuracy-tests-4-gpus (~45.7m wall in
build 83851) into parallelism: 3 via a new opt-in CONFIG_INDICES env in
config_sweep_accuracy_test.sh that selects configs from the chosen array
by zero-based index. Unset means run everything, so the other jobs
sharing the sweep script (tp/dp_ep/sw_attn variants and the AMD mirrors)
are unchanged. CONFIG_EXPECTED_COUNT=7 makes each shard fail loudly if
the hybrid config array changes without rebalancing the index lists.

Buckets are balanced on measured per-config runtime: shard 0 = configs
0,4 (~14.5m), shard 1 = configs 5,6 (~13.2m), shard 2 = configs 1,2,3
(~16.3m); expected wall ~18m incl. ~1.7m install/canary per shard.
Timeout 60 -> 30. The AMD mirror keeps its own unchanged command and is
pinned to parallelism: 1 (stays a single unsharded job; requires the
pipeline generator change in vllm-project/ci-infra#473).

Signed-off-by: Kevin Luu <51931015+khluu@users.noreply.github.com>

Author: khluu

.buildkite/test_areas/disaggregated.yaml

@@ -107,19 +107,27 @@ steps:
 
 - label: Hybrid SSM NixlConnector PD accuracy tests (4 GPUs)
   key: hybrid-ssm-nixlconnector-pd-accuracy-tests-4-gpus
-  timeout_in_minutes: 60
+  timeout_in_minutes: 30
   working_dir: "/vllm-workspace/tests"
   num_devices: 4
+  parallelism: 3
   source_file_dependencies:
     - vllm/distributed/kv_transfer/kv_connector/v1/nixl/
     - tests/v1/kv_connector/nixl_integration/
   commands:
     - bash /vllm-workspace/.buildkite/scripts/install-kv-connectors.sh
-    - HYBRID_SSM=1 bash v1/kv_connector/nixl_integration/config_sweep_accuracy_test.sh
+    # Timing-balanced index buckets over the 7 hybrid_ssm_configs (measured on
+    # build 83851): shard 0 = configs 0,4 (~14.5m), shard 1 = configs 5,6
+    # (~13.2m), shard 2 = configs 1,2,3 (~16.3m). CONFIG_EXPECTED_COUNT makes
+    # the sweep fail loudly if the config array changes without rebalancing.
+    - test "$$BUILDKITE_PARALLEL_JOB" != "0" || HYBRID_SSM=1 CONFIG_EXPECTED_COUNT=7 CONFIG_INDICES=0,4 bash v1/kv_connector/nixl_integration/config_sweep_accuracy_test.sh
+    - test "$$BUILDKITE_PARALLEL_JOB" != "1" || HYBRID_SSM=1 CONFIG_EXPECTED_COUNT=7 CONFIG_INDICES=5,6 bash v1/kv_connector/nixl_integration/config_sweep_accuracy_test.sh
+    - test "$$BUILDKITE_PARALLEL_JOB" != "2" || HYBRID_SSM=1 CONFIG_EXPECTED_COUNT=7 CONFIG_INDICES=1,2,3 bash v1/kv_connector/nixl_integration/config_sweep_accuracy_test.sh
   mirror:
     amd:
       dind: false
       device: mi300_4
+      parallelism: 1
       timeout_in_minutes: 55
       depends_on:
         - image-build-amd

tests/v1/kv_connector/nixl_integration/config_sweep_accuracy_test.sh

@@ -55,6 +55,26 @@ else
   configs=("${tp_configs[@]}")
 fi
 
+# Opt-in CI sharding: CONFIG_INDICES selects a comma-separated subset of the
+# chosen config array by zero-based index; unset means run everything, so all
+# existing callers are unchanged. CONFIG_EXPECTED_COUNT fails the run loudly
+# when the array size changes, instead of letting a newly added config be
+# silently skipped by stale shard index lists.
+if [[ -n "${CONFIG_EXPECTED_COUNT:-}" && "${#configs[@]}" -ne "${CONFIG_EXPECTED_COUNT}" ]]; then
+  echo "Selected config set has ${#configs[@]} entries, expected ${CONFIG_EXPECTED_COUNT}."
+  echo "Rebalance the CONFIG_INDICES shard lists in .buildkite/test_areas for this job."
+  exit 1
+fi
+if [[ -n "${CONFIG_INDICES:-}" ]]; then
+  IFS=',' read -r -a shard_indices <<< "${CONFIG_INDICES}"
+  shard_configs=()
+  for idx in "${shard_indices[@]}"; do
+    shard_configs+=("${configs[$idx]}")
+  done
+  configs=("${shard_configs[@]}")
+  echo "CONFIG_INDICES=${CONFIG_INDICES}: running ${#configs[@]} of the selected configs"
+fi
+
 run_tests() {
   local label=$1
   local extra_args=$2