[V1][PP] Support PP for MultiprocExecutor

[bigPYJ1151]

By offloading MQ reading to a IO thread, MultiprocExecutor can receive multiple batches in flight and support V1 PP seamlessly.

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[comaniac]

Also cc @ruisearch42

[youkaichao]

also cc @njhill and @tlrmchlsmth for the mp-related change.

[bigPYJ1151]

Thanks for @comaniac @ruisearch42 @youkaichao 's comments, I have resolved them all.

I have some initial benchmark results. On a 4xL40 platform:

# TP=2 PP=1
VLLM_USE_V1=1 python3 benchmark_throughput.py --backend=vllm --dataset=./ShareGPT_V3_unfiltered_cleaned_split.json --model=meta-llama/Meta-Llama-3-8B-Instruct --n=1 --num-prompts=1000 --trust-remote-code --disable-log-stats -tp=2 -pp=1 --max-model-len=8192 
Throughput: 5.60 requests/s, 2314.67 total tokens/s, 1110.17 output tokens/s

# TP=1 PP=2
VLLM_USE_V1=1 python3 benchmark_throughput.py --backend=vllm --dataset=./ShareGPT_V3_unfiltered_cleaned_split.json --model=meta-llama/Meta-Llama-3-8B-Instruct --n=1 --num-prompts=1000 --trust-remote-code --disable-log-stats -tp=1 -pp=2 --max-model-len=8192 
Throughput: 5.74 requests/s, 2374.38 total tokens/s, 1138.81 output tokens/s

# TP=1 PP=4
VLLM_USE_V1=1 python3 benchmark_throughput.py --backend=vllm --dataset=./ShareGPT_V3_unfiltered_cleaned_split.json --model=meta-llama/Meta-Llama-3-8B-Instruct --n=1 --num-prompts=1000 --trust-remote-code --disable-log-stats -tp=4 -pp=1 --max-model-len=8192 
Throughput: 7.93 requests/s, 3277.62 total tokens/s, 1572.03 output tokens/s

# TP=1 PP=4
VLLM_USE_V1=1 python3 benchmark_throughput.py --backend=vllm --dataset=./ShareGPT_V3_unfiltered_cleaned_split.json --model=meta-llama/Meta-Llama-3-8B-Instruct --n=1 --num-prompts=1000 --trust-remote-code --disable-log-stats -tp=1 -pp=4 --max-model-len=8192 
Throughput: 9.28 requests/s, 3838.89 total tokens/s, 1841.22 output tokens/s

# TP=2 PP=2
VLLM_USE_V1=1 python3 benchmark_throughput.py --backend=vllm --dataset=./ShareGPT_V3_unfiltered_cleaned_split.json --model=meta-llama/Meta-Llama-3-8B-Instruct --n=1 --num-prompts=1000 --trust-remote-code --disable-log-stats -tp=2 -pp=2 --max-model-len=8192 
Throughput: 9.69 requests/s, 4005.70 total tokens/s, 1921.23 output tokens/s

[WoosukKwon]

Hi @bigPYJ1151, can you please rebase the PR and resolve merge conflicts?

[bigPYJ1151]

Hi @bigPYJ1151, can you please rebase the PR and resolve merge conflicts?

@WoosukKwon Sure, updated, also verified the unit tests. Please take a look :)

[WoosukKwon]

@ruisearch42 @comaniac @youkaichao Can you please take a final look by any chance?

[WoosukKwon]

@bigPYJ1151 I've just started the CI test. Will merge once it becomes green.

[bigPYJ1151]

@WoosukKwon All required became green, please help to merge, thanks :)