[platform] Move get_cu_count to utils

[wangxiyuan]

Purpose

get_cu_count is only used by rocm which is controlled by current_platform.is_rocm already. Remove this redundant platform interface and move it to utils instead.

Test Plan

Test Result

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• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
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[gemini-code-assist]

Code Review

This pull request removes the get_cu_count method from the platform interface, replacing its usages with direct calls to torch.cuda.get_device_properties(0).multi_processor_count. While this simplifies the platform interface, it introduces code duplication and a potential correctness issue in multi-GPU environments due to the hardcoded device ID 0. My review includes critical feedback on this hardcoded device ID and suggests introducing a cached helper function to improve correctness, performance, and maintainability. I've also provided a suggestion to refactor the test code to improve its performance and robustness.

[wangxiyuan]

@DarkLight1337 @youkaichao platform interface cleanup. Need your review. Thanks.

[Yikun]

LGTM, if no more objection I will merge tmr.

[tjtanaa]

LGTM if this is a platform agnostic function.