PR #11859: [XLA:GPU] Add cuDNN Flash Attention Padding Mask Generation & Skipping computation on padded tokens in XLA
PR #11859: [XLA:GPU] Add cuDNN Flash Attention Padding Mask Generation & Skipping computation on padded tokens in XLA

Imported from GitHub PR https://github.com/openxla/xla/pull/11859

* Follow up PR on https://github.com/openxla/xla/pull/11449 and https://github.com/openxla/xla/pull/10232.
* Add cuDNN flash attention padding mask generation in XLA. cuDNN also supports skip computation at padded tokens.
* Add an e2e test to compare padding mask generation with pass padding mask as bias in cuDNN.
Copybara import of the project:

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aab103097be5ab7b23573e605bb8e877e004f323 by cjkkkk <ske@nvidia.com>:

add var_seq

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ba755d7c145a7c54a0cf939ea911d791f6d7f755 by cjkkkk <ske@nvidia.com>:

add uid

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5da56ba30542d75c4e80847788b48df500934579 by cjkkkk <ske@nvidia.com>:

add var seq e2e test

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3a109b8d26c3958557d70c5e0802b0b5e19cb2dc by cjkkkk <ske@nvidia.com>:

fix rebase error

Merging this change closes #11859

FUTURE_COPYBARA_INTEGRATE_REVIEW=https://github.com/openxla/xla/pull/11859 from Cjkkkk:var_seq 3a109b8d26c3958557d70c5e0802b0b5e19cb2dc
