[Bugfix][plugin] fla crash on plugin

[ILikeIneine]

Purpose

There's some problem while supporting fla on plugin.
While importing the fla/ops/utils, it crashed on here.

Since in plugin the device might got their own value (here in vllm-metax is maca) and device_torch_lib still need to be their own. (here in vllm-metax is torch.cuda)

So I use is_cuda_alike and set default value to None on getattr to make some compatibilities for handling the corner cases. The semantics should be consistent with the original code.

Test Plan

Test Result

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[gemini-code-assist]

Code Review

This pull request addresses a crash in Flash-Linear-Attention (FLA) operations when used with plugins. The changes in vllm/model_executor/layers/fla/ops/utils.py are well-reasoned and effective. By leveraging current_platform.is_cuda_alike(), the code now correctly identifies CUDA-compatible platforms (including plugins) and sets the device library appropriately. Adding a None default to getattr is a good defensive measure that prevents crashes on other platforms like CPU, making the utility more robust. The fix is correct and improves the overall stability of FLA operations in diverse environments.

[NickLucche]

I think this looks fine but I don't have the context on fla.
Perhaps @youkaichao can take a quick look at it.

[mgoin]

Looks simple enough to me. I believe the logic is kept the same for Nvidia and AMD, so nothing else changes for Intel or CPU