[Feature] Keep HMA enabled for supported KV connectors

[arpera]

Motivation

vLLM currently disables the hybrid KV cache manager by default whenever kv_transfer_config is set, unless the user explicitly passes --no-disable-hybrid-kv-cache-manager. That is conservative for connectors that do not support HMA, but it also disables HMA for connectors like NixlConnector that already advertise SupportsHMA.

This change preserves the conservative default for unsupported connectors while allowing HMA to stay enabled by default when the selected connector explicitly supports it.

Test Results

Hardware: 4xGB200

Functional

python -m pytest tests/v1/core/test_kv_cache_utils.py::test_hma_not_disabled_for_supported_kv_connector tests/v1/core/test_kv_cache_utils.py::test_hma_disabled_for_unsupported_kv_connector -v — passed.

Performance

Not measured. This only changes the default HMA decision for KV connectors that already declare HMA support.

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[claude]

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[arpera]

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[arpera]

@vadiklyutiy, please, have a look

[orozery]

cc @NickLucche
This will break the multi connector.

[arpera]

@orozery, could you please explain a bit more the issue with multi connector?

[orozery]

@orozery, could you please explain a bit more the issue with multi connector?

See discussion in #39571.
The multi connector currently assumes that HMA will be off by default if kv_transfer_config is present.
Breaking this assumption would lead to a possible case where HMA is enabled though one of the sub-connectors of the multi-connector does not support HMA.

[NickLucche]

Thanks for contributing @arpera.
Let's chat before changing current hma opt-in policy.

[arpera]

Sure, I am open to discussion, if there is any concerns about this change, please, feel free to discuss it there.