[mhlo] Remove called computation and execution thread from async_update and async_done
[mhlo] Remove called computation and execution thread from async_update and async_done
For consistency with HLO counterpart get called computation and execution thread by following the async chain.

FUTURE_COPYBARA_INTEGRATE_REVIEW=https://github.com/openxla/xla/pull/12154 from philipphack:u_fp8_scale_xla c7944fc3b2b7fdc45a06d058dea856fff2bd7e15
