[Core] Add engine option to return only deltas or final output (#7381)

.buildkite/test-pipeline.yaml

@@ -50,6 +50,7 @@ steps:
   - tests/worker
   commands:
   - pytest -v -s async_engine # Async Engine
+  - NUM_SCHEDULER_STEPS=4 pytest -v -s async_engine/test_async_llm_engine.py
   - pytest -v -s test_inputs.py
   - pytest -v -s multimodal
   - pytest -v -s test_utils.py # Utils