[V1] Support DP with Ray

[ruisearch42]

This PR adds support for DP with Ray, with support for multi-node and API server scale out.

We reuse ZMQ communication mechanism between frontend and engine cores, as in #15977 , and the same API server scale out mechanism, as in #17546

Main differences from those PRs:

• The handshake between frontend and engine cores are greatly simplified, thanks to the Ray API
• We can launch all DP ranks just on the head node

Examples

This will run DP=4 on the head node.

# Head node  (with ip address 10.99.48.128)
vllm serve $MODEL --data-parallel-size 4 --data-parallel-size-local 4 \
                  --data-parallel-address 10.99.48.128 --data-parallel-rpc-port 13345
                  --data-parallel-backend ray

This will run DP=4 with DP ranks 0 and 1 on the head node and ranks 2 and 3 on other nodes.

# Head node  (with ip address 10.99.48.128)
vllm serve $MODEL --data-parallel-size 4 --data-parallel-size-local 2 \
                  --data-parallel-address 10.99.48.128 --data-parallel-rpc-port 13345
                  --data-parallel-backend ray

This will run DP=4 with only the API server on the head node and all engines other nodes:

# Head node  (with ip address 10.99.48.128)
vllm serve $MODEL --data-parallel-size 4 --data-parallel-size-local 0 \
                  --data-parallel-address 10.99.48.128 --data-parallel-rpc-port 13345
                  --data-parallel-backend ray

Design

See the following illustration. DP Coordinator is omitted, but is the same as #17546 .

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[kouroshHakha]

Overall, looks good. There are some comments that need to be addressed and some follow ups that are not blockers.

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @ruisearch42.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

[njhill]

Thanks @ruisearch42 this looks pretty clean

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @ruisearch42.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

[njhill]

Thanks @ruisearch42

Curious why all of the new test dependencies, I thought various other tests already used ray.

[ruisearch42]

Curious why all of the new test dependencies, I thought various other tests already used ray.

list_nodes() is part of Ray state API, and requires ray[default].