[core] LLM.collective_rpc interface and RLHF example (#12084)

Signed-off-by: youkaichao <youkaichao@gmail.com>

.buildkite/test-pipeline.yaml

@@ -126,11 +126,15 @@ steps:
   - tests/distributed
   - tests/spec_decode/e2e/test_integration_dist_tp4
   - tests/compile
+  - examples/offline_inference/rlhf.py
   commands:
   - pytest -v -s distributed/test_utils.py
   - pytest -v -s compile/test_basic_correctness.py
   - pytest -v -s distributed/test_pynccl.py
   - pytest -v -s spec_decode/e2e/test_integration_dist_tp4.py
+  # TODO: create a dedicated test section for multi-GPU example tests
+  # when we have multiple distributed example tests
+  - python3 ../examples/offline_inference/rlhf.py
 
 - label: Metrics, Tracing Test # 10min
   num_gpus: 2