[UX] Format attention backend log line

[MatthewBonanni]

Purpose

Clean up attention backend log line to match MoE

Main:

(EngineCore_DP0 pid=2126810) INFO 02-02 17:08:08 [cuda.py:364] Using FLASHMLA_SPARSE attention backend out of potential backends: ('FLASHMLA_SPARSE',)
(EngineCore_DP0 pid=2126810) INFO 02-02 17:08:08 [fp8.py:329] Using FLASHINFER_CUTLASS Fp8 MoE backend out of potential backends: ['AITER', 'FLASHINFER_TRTLLM', 'FLASHINFER_CUTLASS', 'DEEPGEMM', 'BATCHED_DEEPGEMM', 'TRITON', 'BATCHED_TRITON', 'MARLIN'].

PR:

(EngineCore_DP0 pid=2133585) INFO 02-02 17:11:37 [cuda.py:364] Using FLASHMLA_SPARSE attention backend out of potential backends: ['FLASHMLA_SPARSE'].
(EngineCore_DP0 pid=2133585) INFO 02-02 17:11:37 [fp8.py:329] Using FLASHINFER_CUTLASS Fp8 MoE backend out of potential backends: ['AITER', 'FLASHINFER_TRTLLM', 'FLASHINFER_CUTLASS', 'DEEPGEMM', 'BATCHED_DEEPGEMM', 'TRITON', 'BATCHED_TRITON', 'MARLIN'].

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

Code Review

This pull request updates the formatting of a log message for the attention backend selection to improve consistency with other log messages in the codebase. The change is minor, replacing tuple-style output with a bracketed list and adding a period. The implementation is correct and achieves the intended cosmetic improvement. The code looks good, and I have no further comments.