[CI] Shard Model Executor and L4 Pipeline + Context Parallelism automatically (#61202)

Signed-off-by: khluu <khluu000@gmail.com>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

.buildkite/test_areas/distributed.yaml

@@ -641,6 +641,7 @@ steps:
 
 - label: ":nvidia: (L4) Pipeline + Context Parallelism"
   key: pipeline-context-parallelism-4-gpus
+  automatic_shard: true
   timeout_in_minutes: 55
   working_dir: "/vllm-workspace/tests"
   device: l4

.buildkite/test_areas/model_executor.yaml

@@ -5,6 +5,7 @@ steps:
 - label: ":nvidia: (H200 MIG 35GB) Model Executor"
   device: h200_35gb
   key: model-executor
+  automatic_shard: true
   timeout_in_minutes: 60
   source_file_dependencies:
   - vllm/engine/arg_utils.py