[P/D] Support NIXL connector to disconnect during a clean shutdown

[chaunceyjiang]

Essential Elements of an Effective PR Description Checklist

• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
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Part of #22295

Refer https://github.com/ai-dynamo/nixl/blob/main/examples/python/nixl_api_example.py#L205-L211

Purpose

1）Support disconnecting from clean shutdown

Test Result

[gemini-code-assist]

Code Review

This pull request refactors the cleanup logic in NixlConnectorWorker to ensure resources are released on clean shutdown by using atexit. However, the current implementation can lead to the shutdown logic being called multiple times, which could cause errors. My review includes a critical fix to make the shutdown process idempotent, preventing potential crashes during program exit.

[chaunceyjiang]

/cc @robertgshaw2-redhat @njhill PTAL.

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[njhill]

Thanks for this @chaunceyjiang!