CI test linux://python/ray/llm/tests:batch/gpu/stages/test_vllm_engine_stage is consistently_failing

[aslonnie]

CI test linux://python/ray/llm/tests:batch/gpu/stages/test_vllm_engine_stage is consistently_failing. Recent failures:
- https://buildkite.com/ray-project/postmerge/builds/14590#019ab7fe-672b-422c-bdf6-66df3ee7e041
- https://buildkite.com/ray-project/postmerge/builds/14590#019ab7b5-b7bf-48be-8717-d96d98f1174b

DataCaseName-linux://python/ray/llm/tests:batch/gpu/stages/test_vllm_engine_stage-END
Managed by OSS Test Policy

[aslonnie]

This test is now considered as flaky because it has been failing on postmerge for too long. Flaky tests do not run on premerge.

[aslonnie]

Test passed on latest run: https://buildkite.com/ray-project/postmerge/builds/14657#019ac723-f7ee-48fd-929e-dab1671f0162