[BugFix][KVConn] Fix use of get_required_kvcache_layout

[njhill]

Follow-on from #22663.

cc @NickLucche

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[gemini-code-assist]

Code Review

This pull request fixes an issue in get_required_kvcache_layout where it was being called on the base class instead of the specific connector class. The change in base.py correctly adds a safeguard to prevent this in the future. However, the fix in multi_connector.py introduces a new issue by not providing the correct vllm_config to the sub-connectors' get_required_kvcache_layout method. I've provided a critical comment with a suggested fix for this.

[gemini-code-assist]

This change correctly calls get_required_kvcache_layout on the specific connector_cls. However, it passes the original vllm_config from the MultiConnector, not a config tailored for the sub-connector. If a sub-connector's get_required_kvcache_layout implementation relies on its specific kv_transfer_config (e.g., from kv_connector_extra_config), it will receive the wrong configuration.

This is inconsistent with the pattern used in MultiConnector.__init__, which creates a temporary config for each sub-connector. To ensure correctness and consistency, a temporary vllm_config should be created for each sub-connector within the loop, similar to the __init__ method.

        temp_vllm_config = copy.copy(vllm_config)
        for ktc in ktcs:
            kv_transfer_config = KVTransferConfig(**ktc)
            temp_vllm_config.kv_transfer_config = kv_transfer_config
            connector_cls = KVConnectorFactory.get_connector_class(
                kv_transfer_config)
            required_kvcache_layout = (
                connector_cls.get_required_kvcache_layout(temp_vllm_config))
            if required_kvcache_layout is not None:
                layouts.add(required_kvcache_layout)

[NickLucche]

Thanks for spotting this Nick!

[NickLucche]

btw to expand on this PR, this will also fix heteroTP in a multiconnector scenario, among other cases.