[ROCm] Make amdsmi import optional for other platforms

[DarkLight1337]

This fixes broken CI on other platforms: https://buildkite.com/vllm/ci/builds/13649#0195176d-c5df-4dfc-91dd-4e1b7b2bc14a

[github-actions]

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[youkaichao]

please test locally to see if fail to import (amdsmi_get_gpu_asic_info, amdsmi_get_processor_handles, amdsmi_init, amdsmi_shut_down) can still pass the test.

[DarkLight1337]

I have tested test_attention_selector.py, test_mha_attn.py and test_rocm_attention_selector.py and they all pass.