Introduce RayPPCommunicator for ray-based PP

[ruisearch42]

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• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.

Purpose

Currently, ray-based PP uses a cupy/nccl communicator created separately from other communicators in vLLM. This caused inconsistencies and potential conflicts.

This PR introduces RayPPCommunicator, which wraps around vLLM _PP communicator so that the same underlying communicator is used.

This is the major change needed for #19758
and an alternative to #19816

Test Plan

Test with test_pipeline_parallel.py

Test Result

Passed

(Optional) Documentation Update

[github-actions]

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[gemini-code-assist]

Code Review

This pull request introduces RayPPCommunicator to integrate vLLM's pipeline parallelism with Ray's Compiled DAG by wrapping the existing vLLM _PP communicator. The changes are logical and well-structured. My review focuses on the new ray_communicator.py file, where I've identified a couple of high-severity maintainability issues: a copy-paste error in error messages and the use of a private Ray API attribute which could be brittle. Addressing these will improve the quality of this new component.

[ruisearch42]

@stephanie-wang @youkaichao Could you please help review, thanks

[stephanie-wang]

Do we need this flag? Can we just always use vLLM's PP communicator?

[ruisearch42]

The change in this PR is by default on. Wanted to have this config guard for a while so that users can fall back if needed.
This will be cleaned up altogether after it is rolled out for a while.

[ruisearch42]

Hi @stephanie-wang , I've addressed your comments and full CI passed. Could you take another look?

[carmark]

@ruisearch42 Is there any perf improvement with this pr? Thanks.