Remove virtual engine handling

[WoosukKwon]

Summary

• remove the virtual engine field from the forward context helpers
• simplify KV cache access across attention layers and KV connectors to use a single cache instance
• update KV connector tests to align with the new forward context interface

Testing

• python -m pytest tests/v1/kv_connector/unit/test_lmcache_integration.py::test_forward_context_interface (fails: ModuleNotFoundError: tblib)
• python -m compileall tests/v1/kv_connector/unit/test_nixl_connector.py
• python -m compileall tests/v1/kv_connector/unit/test_offloading_connector.py tests/v1/kv_connector/unit/test_decode_bench_connector.py tests/v1/kv_connector/unit/test_lmcache_integration.py

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Codex Task

[chatgpt-codex-connector]

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[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @WoosukKwon.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

[gemini-code-assist]

Code Review

This pull request removes the virtual_engine field and its handling throughout the codebase. The changes simplify KV cache access by assuming a single cache instance instead of a list indexed by virtual_engine. The modifications are consistently applied across various components, including attention layers, KV connectors, model implementations, and tests. The refactoring is clean and aligns with the goal of simplifying the KV cache management. I have reviewed the changes and found no issues.

[XingLiu1]

Hi maintainers, quick question about scope:

Does this PR also cover the TODO in
https://github.com/vllm-project/vllm/blob/main/vllm/v1/worker/gpu/kv_connector.py#L72
("sort out KV Connectors' use of forward_context", introduced in #32742)?

Right now pre_forward() falls back to set_forward_context(None, ...) before
start_load_kv(get_forward_context()) when no forward context is active.
Should that behavior be considered resolved by this PR, or should it be tracked as a separate follow-up issue?