[KVConnector] Always call connector clear_metadata() at end of step

[njhill]

• Move call to clear_metadata() from model runner into calling method in worker, so that it's always the last connector method called per step.
• Reset the connector _connector_metadata field to None rather than an empty KVConnectorMetadata superclass; make that class abstract.
• Dedup the aggregation logic in multiproc_executor.py.

This PR is mostly changes suggested by @sdavidbd in #19555 (comment).

cc @orozery

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[gemini-code-assist]

Summary of Changes

Hello @njhill, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request refines the management of KVConnector metadata by ensuring the clear_metadata() method is consistently called at the appropriate stage of the execution pipeline. It relocates this crucial cleanup operation to guarantee its execution at the end of each processing step, improving the reliability of the KV transfer mechanism. Additionally, it includes minor refactoring for improved code clarity and maintainability.

Highlights

• KVConnector Metadata Management: The KVConnectorMetadata class is now an Abstract Base Class (ABC), and the _connector_metadata attribute within KVConnectorBase_V1 is explicitly typed as Optional and initialized to None. This enhances type safety and clarifies the lifecycle of metadata objects.
• Relocation of clear_metadata() Call: The call to get_kv_transfer_group().clear_connector_metadata() has been moved from vllm/v1/worker/gpu_model_runner.py to vllm/v1/worker/gpu_worker.py. This ensures the KVConnector metadata is consistently cleared at the end of every processing step within the execute_model method, aligning with the intended cleanup timing.
• Code Refactoring for Aggregation Logic: In vllm/v1/executor/multiproc_executor.py, the _aggregate_workers_output method has been refactored. A new helper function, update_finished_set, was introduced to deduplicate and streamline the logic for processing and updating finished_sending and finished_recving sets.

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[gemini-code-assist]

Code Review

This pull request correctly moves the clear_connector_metadata() call to a more reliable location at the end of the worker step, ensuring it's always executed. The changes in base.py to make KVConnectorMetadata an ABC and use Optional typing improve code clarity and correctness. The refactoring in multiproc_executor.py to eliminate duplicated code is also a good improvement for maintainability. Overall, the changes are well-implemented and address the intended issue effectively.

[KuntaiDu]

LGTM!