[Bugfix] Fix spec decoding when seed is none in a batch

[wallashss]

The issue was in the rejection_sampler in a custom _multinomial function. When a batch contains a seed as None, the calculation fails because the access with non-spec indices does not change in place. So, I changed to every request to be calculated the same way as seeded using ranges.

Concise script to repro the bug

from vllm import EngineArgs, LLMEngine
from vllm.sampling_params import SamplingParams
from vllm.platforms import current_platform

model='meta-llama/Llama-3.1-8B-Instruct'
speculative_model='ibm-fms/llama3-8b-accelerator'

current_platform.seed_everything(0)
engine_args = EngineArgs(model=model,
                            gpu_memory_utilization=0.95, 
                            use_v2_block_manager=True,
                            enforce_eager=True,
                            speculative_model=speculative_model, 
                            num_speculative_tokens=4
                            )
engine = LLMEngine.from_engine_args(engine_args)

prompts = [
    # Expected: "I will never miss any tokens."...
    # Wrong: "I!!!!!!!! will"...
    ('Write the following phrase exactly 20 times: I will never miss any tokens.\n1: I will never miss any tokens.\n2: ', 
     SamplingParams(temperature=0.1, stop=['20:'], seed=None, min_tokens=10, max_tokens=10, logprobs=1)),
    ('How to make pizza??? ', 
     SamplingParams(temperature=0.1, seed=1, logprobs=1)),
]
for i, (prompt, params) in enumerate(prompts):
    engine.add_request(
        f"request-id-{i}",
        prompt,
        params,
    )

while engine.has_unfinished_requests():
    res = engine.step()
    if not res:
        continue
    for r in res:
        if r.finished:
            print(r.outputs[0].text)


print('_______')
current_platform.seed_everything(0)
# Patch to all request be seed None
prompts[1][1].seed = None 
for i, (prompt, params) in enumerate(prompts):
    engine.add_request(
        f"request-id-{i+2}",
        prompt,
        params,
    )
    
while engine.has_unfinished_requests():
    res = engine.step()
    if not res:
        continue
    for r in res:
        if r.finished:
            print(r.outputs[0].text)

FIX #9441 (link existing issues this PR will resolve)

[github-actions]

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[joerunde]

Looks like a network timeout on the tests :(

[mgoin]

I retried the failing builds, let's see if they can be green this far back

[wallashss]

Thank you very much @mgoin !