[XLA:GPU] Disable cuDNN FMHA by default.
[XLA:GPU] Disable cuDNN FMHA by default.

cuDNN FMHA dispatches pattern-matched regions to a FlashAttention kernel by
default. FlashAttention does not preserve numerics, and thus an illegal
optimization to have on by default.

FUTURE_COPYBARA_INTEGRATE_REVIEW=https://github.com/openxla/xla/pull/12193 from openxla:move_fmha_workspace ad747544396d4920e60946d7a0f00300f47e07d2
