OffloadingConnector: Fix GPU block tracking bug

[orozery]

This PR fixes a bug in the offloading connector that may result in incorrect GPU block tracking per request.
It occurs when blocks cannot be allocated on the offloaded medium (prepare_store fails), and the scheduler output has multiple requests, some of them with new GPU block IDs. Before this PR, the connector simply returned without processing the rest of the requests, and their GPU block IDs.

This resulted in a crash of the engine core:

^[[1;36m(EngineCore_0 pid=271)^[[0;0m ERROR 09-21 17:33:21 [core.py:714] EngineCore encountered a fatal error.
^[[1;36m(EngineCore_0 pid=271)^[[0;0m ERROR 09-21 17:33:21 [core.py:714] Traceback (most recent call last):
^[[1;36m(EngineCore_0 pid=271)^[[0;0m ERROR 09-21 17:33:21 [core.py:714]   File "/workspace/vllm/vllm/v1/engine/core.py", line 705, in run_engine_core
^[[1;36m(EngineCore_0 pid=271)^[[0;0m ERROR 09-21 17:33:21 [core.py:714]     engine_core.run_busy_loop()
^[[1;36m(EngineCore_0 pid=271)^[[0;0m ERROR 09-21 17:33:21 [core.py:714]   File "/workspace/vllm/vllm/v1/engine/core.py", line 732, in run_busy_loop
^[[1;36m(EngineCore_0 pid=271)^[[0;0m ERROR 09-21 17:33:21 [core.py:714]     self._process_engine_step()
^[[1;36m(EngineCore_0 pid=271)^[[0;0m ERROR 09-21 17:33:21 [core.py:714]   File "/workspace/vllm/vllm/v1/engine/core.py", line 758, in _process_engine_step
^[[1;36m(EngineCore_0 pid=271)^[[0;0m ERROR 09-21 17:33:21 [core.py:714]     outputs, model_executed = self.step_fn()
^[[1;36m(EngineCore_0 pid=271)^[[0;0m ERROR 09-21 17:33:21 [core.py:714]                               ^^^^^^^^^^^^^^
^[[1;36m(EngineCore_0 pid=271)^[[0;0m ERROR 09-21 17:33:21 [core.py:714]   File "/workspace/vllm/vllm/v1/engine/core.py", line 290, in step
^[[1;36m(EngineCore_0 pid=271)^[[0;0m ERROR 09-21 17:33:21 [core.py:714]     scheduler_output = self.scheduler.schedule()
^[[1;36m(EngineCore_0 pid=271)^[[0;0m ERROR 09-21 17:33:21 [core.py:714]                        ^^^^^^^^^^^^^^^^^^^^^^^^^
^[[1;36m(EngineCore_0 pid=271)^[[0;0m ERROR 09-21 17:33:21 [core.py:714]   File "/workspace/vllm/vllm/v1/core/sched/scheduler.py", line 589, in schedule
^[[1;36m(EngineCore_0 pid=271)^[[0;0m ERROR 09-21 17:33:21 [core.py:714]     meta = self.connector.build_connector_meta(scheduler_output)
^[[1;36m(EngineCore_0 pid=271)^[[0;0m ERROR 09-21 17:33:21 [core.py:714]            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
^[[1;36m(EngineCore_0 pid=271)^[[0;0m ERROR 09-21 17:33:21 [core.py:714]   File "/workspace/vllm/vllm/distributed/kv_transfer/kv_connector/v1/offloading_connector.py", line 101, in build_connector_meta
^[[1;36m(EngineCore_0 pid=271)^[[0;0m ERROR 09-21 17:33:21 [core.py:714]     return self.connector_scheduler.build_connector_meta(scheduler_output)
^[[1;36m(EngineCore_0 pid=271)^[[0;0m ERROR 09-21 17:33:21 [core.py:714]            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
^[[1;36m(EngineCore_0 pid=271)^[[0;0m ERROR 09-21 17:33:21 [core.py:714]   File "/workspace/vllm/vllm/distributed/kv_transfer/kv_connector/v1/offloading_connector.py", line 323, in build_connector_meta
^[[1;36m(EngineCore_0 pid=271)^[[0;0m ERROR 09-21 17:33:21 [core.py:714]     reqs_to_store=self._get_reqs_to_store(scheduler_output))
^[[1;36m(EngineCore_0 pid=271)^[[0;0m ERROR 09-21 17:33:21 [core.py:714]                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
^[[1;36m(EngineCore_0 pid=271)^[[0;0m ERROR 09-21 17:33:21 [core.py:714]   File "/workspace/vllm/vllm/distributed/kv_transfer/kv_connector/v1/offloading_connector.py", line 305, in _get_reqs_to_store
^[[1;36m(EngineCore_0 pid=271)^[[0;0m ERROR 09-21 17:33:21 [core.py:714]     GPULoadStoreSpec(block_ids[gpu_block_idx + i]))
^[[1;36m(EngineCore_0 pid=271)^[[0;0m ERROR 09-21 17:33:21 [core.py:714]                      ~~~~~~~~~^^^^^^^^^^^^^^^^^^^
^[[1;36m(EngineCore_0 pid=271)^[[0;0m ERROR 09-21 17:33:21 [core.py:714] IndexError: list index out of range

[gemini-code-assist]

Code Review

This pull request addresses a critical bug in the OffloadingConnector that could cause the engine to crash. The issue occurred when preparing to store K/V cache blocks for offloading failed for one request, causing the system to stop processing subsequent requests in the same batch. This resulted in inconsistent state for GPU block tracking and an IndexError. The fix correctly replaces a break with a continue statement, ensuring that even if one request fails to offload, other requests in the batch are still processed correctly. The change is small, targeted, and effectively resolves the described crash. The logic is sound and I see no further issues.

[orozery]

@njhill