[V1] Fix: make sure k_index is int64 for apply_top_k_only

[b8zhong]

The sampler in vLLM fails with

RuntimeError: gather(): Expected dtype int64 for index

because the index tensor for torch.gather isn’t cast to torch.int64, resulting in a dtype mismatch error. this cast should be safe.

Bug reproduction

python benchmarks/benchmark_serving.py \
    --backend deepspeed-mii \
    --model NousResearch/Meta-Llama-3-8B-Instruct \
    --host 127.0.0.1 \
    --port 8000 \
    --dataset-name random \
    --num-prompts 100 \
    --request-rate 10

Similar to #15065, #15049

cc @houseroad I think you would probably know this. What do you think?

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[WoosukKwon]

LGTM. @njhill An idea why this bug has not been detected in tests?

[WoosukKwon]

@njhill It's because the test uses k = torch.randint, and torch.randint outputs int64 tensors

vllm/tests/v1/sample/test_topk_topp_sampler.py

Line 21 in 6efb195

k = torch.randint(1, 10, (BATCH_SIZE, ), generator=generator)

[njhill]

Thanks for fixing @b8zhong