PR #11717: [XLA:GPU] Default to flash attention and remove mask input in cuDNN
PR #11717: [XLA:GPU] Default to flash attention and remove mask input in cuDNN

Imported from GitHub PR https://github.com/openxla/xla/pull/11717

* Default to flash attention as it is high performant and maintained actively by cuDNN, remove old fused attn.
  * Remove lowering to fused attn in rewriter
  * Remove cudnn graph generation
* Remove mask input as it is only doing multiply instead of masking with -inf, give incorrect results. Also cuDNN does not support this anymore, mask should be combined with bias. This is follow up on https://github.com/openxla/xla/pull/11444.
  * Remove mask logic in rewriter
  * Remove mask buffer/descriptor in thunk
* Remove bmm1-bmm2 pattern as it is not support by flash attention. Modified related rewriter test to use bmm1-softmax - bmm2.

Current pattern:
bmm1 - (scale) - (bias) - softmax - (dropout) - bmm2.
Copybara import of the project:

--
552b4a3387c6d5b2b5adcf31b6f44cc858387b23 by cjkkkk <ske@nvidia.com>:

remove fused attn

--
13b683bf923e6fe344f879f913ae6ce41334eeb2 by cjkkkk <ske@nvidia.com>:

remove mask and bmm1-bmm2 pattern

--
9e843dd66c8f7d51d239b433e1b9bc329afee90d by cjkkkk <ske@nvidia.com>:

rm unused vari

--
1104df540e9196b34d9e61e679d88260151728d5 by cjkkkk <ske@nvidia.com>:

remove fused attn cudnnv version check and update flash attn cudnn version check

--
b020cb8e91d8f7b834645ff815c38c4798174857 by cjkkkk <ske@nvidia.com>:

remove mask related cudnnfmhakind&descriptor&buffer

--
ff1952faa460eecfca62660a5c34ea6fa3c2dfd4 by cjkkkk <ske@nvidia.com>:

rename hlo_string to shorter name

Merging this change closes #11717

FUTURE_COPYBARA_INTEGRATE_REVIEW=https://github.com/openxla/xla/pull/11717 from Cjkkkk:remove_fused_attn_and_mask ff1952faa460eecfca62660a5c34ea6fa3c2dfd4
