PR #11449: [XLA:GPU] replace is_causal_mask with enum mask_type to support more cudnn mask generation
PR #11449: [XLA:GPU] replace is_causal_mask with enum mask_type to support more cudnn mask generation

Imported from GitHub PR https://github.com/openxla/xla/pull/11449

* replace is_causal_mask in backend_config with enum mask_type.
* cudnn also supports padding mask and alibi mask.
 
Copybara import of the project:

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5780ffd6622978c0a173d90cc562d32f354bfbfd by cjkkkk <ske@nvidia.com>:

replace is_causal_mask with mask_type to support more mask

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73555dba8f8cc1319cf75ed9cf8fe49ad88948bd by cjkkkk <ske@nvidia.com>:

add newline

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2fed993035a2e5c17c990b60b7268ea9dbaeb7e5 by cjkkkk <ske@nvidia.com>:

add is_causal_mask back in backend_config

Merging this change closes #11449

FUTURE_COPYBARA_INTEGRATE_REVIEW=https://github.com/openxla/xla/pull/11449 from Cjkkkk:cudnn_mask_type 2fed993035a2e5c17c990b60b7268ea9dbaeb7e5
