[misc] fix: fix device

[vermouth1992]

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What does this PR do?

Currently, the device to run on depends on whether is_cuda_available is True on the driver process. However, the driver process may be a CPU process that can't see cuda devices even when cuda devices are available. Thus, it's not appropriate to use is_cuda_available to set the device. Instead, we should set the device explicitly.

In the future, we may have a ray cluster with both NPU and GPU, and we can use different devices for different workloads. Thus, setting device explicitly would be a better choice in the long run.

Why CI can't trigger this problem: because we directly run python3 xxx on CI machine instead of using a standard ray cluster that has dedicated CPUs for head. CI machines all have GPUs.

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• Training: [Note which backend this PR will affect: FSDP, Megatron, both, or none]
• Inference: [Note which backend this PR will affect: vLLM, SGLang, both, or none]

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[sunyi0505]

I think it is necessary to modify the test cases in the verl.tests.npu by adding trainer. device=npu