message: Set async as default for TFRT runtime
diff: @@ -427,7 +427,7 @@ class Context(object):
  raise ValueError(
  "execution_mode should be None/SYNC/ASYNC. Got %s" % execution_mode)
  if execution_mode is None:
  - execution_mode = SYNC
  + execution_mode = ASYNC if is_tfrt_enabled() else SYNC
  self._default_is_async = execution_mode == ASYNC
  self._lazy_remote_inputs_copy = None
  self._use_tfrt = is_tfrt_enabled()
  