PR #12005: [XLA:GPU] Fix cuDNN flash attention unit test failure with cuDNN 9.0
PR #12005: [XLA:GPU] Fix cuDNN flash attention unit test failure with cuDNN 9.0

Imported from GitHub PR https://github.com/openxla/xla/pull/12005

* Use `build_plans` instead of `build_plan_at_index` for flash attention as cuDNN 9.0 could have more than 1 plan and some plans cant be built.
Copybara import of the project:

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5379425cce6606435c84219eee3d4ca4f98fc5ab by cjkkkk <ske@nvidia.com>:

use build_plans as cudnn 9 has more than 1 plan

Merging this change closes #12005

FUTURE_COPYBARA_INTEGRATE_REVIEW=https://github.com/openxla/xla/pull/12005 from Cjkkkk:cudnn_9_0_fa_fix 5379425cce6606435c84219eee3d4ca4f98fc5ab
