message: Make context.execution_mode(None) a no-op
diff: @@ -1905,6 +1905,9 @@ def set_execution_mode(mode):
  @tf_contextlib.contextmanager
  def execution_mode(mode):
  """Context manager for setting execution mode for current thread."""
  + if mode is None:
  + yield
  + else:
  ctx = context()
  executor_new = executor.new_executor(mode == ASYNC)
  executor_old = ctx.executor
  