fix Flash Attention kernel not found issue.

[lzbgt]

since sdk_kernel in backends.cuda is deprecated by torch.nn.attention.sdpa_kernel.

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[catwell]

This does not fix the FlashAttention not found issue, it just silents the log because it will fallback to the next kernel.

[lzbgt]

"flash attention not found" should not be corrected by this.
user should check flash attention installed/setup correctly