[feature][WIP] Enable KV Offload for DeepSeek V4 model

[foraxe]

[feature][WIP] Enable KV Offload for DeepSeek V4 model

Summary

This PR makes the v1 OffloadingConnector advertise SupportsHMA and handle
the scheduler's all-KV-group request-finish callback. This is the remaining
connector facade needed for grouped KV offload support when the scheduler passes
tuple[list[int], ...] block IDs for multiple KV cache groups.

The implementation is backend-neutral. It does not add Ascend imports,
torch_npu, DSv4-specific branches, or VLLM_ASCEND_* gates.

Existing generic grouped-KV pieces in this branch

• SupportsHMA is already defined in
  vllm/distributed/kv_transfer/kv_connector/v1/base.py.
• GPULoadStoreSpec already has typed group_sizes and block_indices fields.
• offloading/scheduler.py already tracks RequestOffloadState per KV group.
• The offloading scheduler already uses make_offload_key(..., group_idx) for
  group-aware offload keys.
• Load/store metadata already carries grouped GPU block IDs through
  group_sizes and block_indices.

Changes

• Make OffloadingConnector inherit SupportsHMA.
• Add OffloadingConnector.request_finished_all_groups(...) and delegate to the
  existing scheduler finish path.
• Widen the offloading scheduler finish type annotation so the connector can pass
  either the legacy single-group list or the HMA all-group tuple.
• Add unit coverage for the connector facade so the class is recognized as
  HMA-capable and forwards all-group block IDs unchanged.

Validation

Intended focused tests:

pytest -q tests/v1/kv_connector/unit/offloading_connector/test_connector.py
pytest -q tests/v1/kv_connector/unit/offloading_connector/test_scheduler.py

In this local environment, pytest collection currently requires missing optional
test/runtime dependencies (tblib, then gguf on direct import). Syntax-level
checks were used locally until the full vLLM test environment is available.

Follow-up

The hardware backend remains out of scope for this PR. DSv4 compressed KV
registration, NPU-visible host memory, and A3 launch/runtime validation belong
in the paired vllm-ascend change.

[claude]

Claude Code Review

This pull request is from a fork — automated review is disabled. A repository maintainer can comment @claude review to run a one-time review.

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[gemini-code-assist]

Code Review

This pull request implements SupportsHMA for the OffloadingConnector and introduces the request_finished_all_groups method, which delegates request completion handling to the connector scheduler. The scheduler's request_finished method was also updated to accept a tuple of block ID lists, and corresponding unit tests were added to verify these changes. I have no feedback to provide.

[markmc]

AIUI, @orozery was waiting on #39186 and now #41228 before enabling SupportsHMA in one final PR