[XLA:GPU] Flush out any pending work before starting autotune

The autotune code assumes a clean slate, but there might be things from
previous program executions still pending on the streams owned by the executor.
Do a full host-device sync before autotuning to flush out any pending work.

I'm still somewhat confused on how autotune can interfere with other buffers.
There might be more things going wrong ...

PiperOrigin-RevId: 211369162