message: Throw error if context exists but graph is empty when enabling eager mode.
diff: @@ -4703,6 +4703,9 @@ def enable_eager_execution(config=None, device_policy=None):
  " policy: %s." % (config, context._context._config,
  device_policy,
  context._context._device_policy))
  + else:
  + raise ValueError(
  + "tfe.enable_eager_execution has to be called at program startup.")
  def eager_run(main=None, argv=None):
  