[Core] Update Ascend NPU environment variable and test cases

[liuxsh9]

Why are these changes needed?

With the updates to the NPU foundational software CANN, frameworks like PyTorch(NPU) and MindSpore can now detect the ASCEND_RT_VISIBLE_DEVICES environment variable and intelligently configure devices. This is the recommended usage by the official documentation.

Before:

@ray.remote(resources={"NPU":1})
def Func():
    npu_ids = ray.get_runtime_context().get_accelerator_ids()["NPU"]
    torch_npu.npu.set_device(int(npu_ids[0]))
    model.to("npu")

After:

@ray.remote(resources={"NPU":1})
def Func():
    model.to("npu")

Therefore, we update the environment variable and modify the test cases accordingly.

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    method in Tune, I've added it in doc/source/tune/api/ under the
    corresponding .rst file.
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