[serve.llm] release tests for 1p1d

[lk-chen]

Why are these changes needed?

Enable release test for P/D disagg. implemented in #53092. This PR includes:

• update of ray-llm image, that installs UCX/NIXL, and set proper env vars moved to [ray-llm] Update ray-llm docker to install UCX/NIXL #53377
• for this specific use case, install special version of vLLM that includes necessary feature
• fix/add tests

Related issue number

RFC #53257
Ray-LLM Roadmap #51313

Checks

• I've signed off every commit(by using the -s flag, i.e., git commit -s) in this PR.
• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.
  • I've added any new APIs to the API Reference. For example, if I added a
    method in Tune, I've added it in doc/source/tune/api/ under the
    corresponding .rst file.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(

[kouroshHakha]

some minor comments, I'll review once more when tests pass and this is rebased.

[kouroshHakha]

LGTM

[kouroshHakha]

@lk-chen conflict?

[aslonnie]

approval for the byod script. nice job!