message: [XLA:Python] Enable asynchronous dispatch on the CPU backend.
  This has the effect of running compute kernels asynchronously in a separate thread, returning control to Python as soon as operations are dispatched.
diff: @@ -159,10 +159,18 @@ class LocalBackend(Backend):
  return executable.ExecutePerReplica(per_replica_args)
  +def _cpu_backend_factory():
  + return LocalBackend(platform='cpu', xla_platform_id='Host', asynchronous=True)
  +
  +
  +def _gpu_backend_factory():
  + return LocalBackend(platform='gpu', xla_platform_id='CUDA')
  +
  +
  # Backend factories, keyed by user-visible name, in increasing priority order.
  _local_backend_factories = collections.OrderedDict([
  - ('cpu', lambda: LocalBackend(platform='cpu', xla_platform_id='Host')),
  - ('gpu', lambda: LocalBackend(platform='gpu', xla_platform_id='CUDA')),
  + ('cpu', _cpu_backend_factory),
  + ('gpu', _gpu_backend_factory),
  ])
  