[BE] Make `USE_FLASH_ATTENTION` private (#97579)
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### <samp>🤖 Generated by Copilot at b07152e</samp>
This pull request refactors the CMake configuration to enable the `USE_FLASH_ATTENTION` feature for the `torch_cuda` target only, using a target-specific macro. This avoids conflicts with other libraries that also use this feature, such as fairseq.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/97579
Approved by: https://github.com/kit1980
