[pjrt] Clean up PjRtFuture documentation and internal APIs
[pjrt] Clean up PjRtFuture documentation and internal APIs

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