message: Only start_step/end_step on GradientTape if executing eagerly.
  This prevents creating a context where none is required.
diff: @@ -725,6 +725,8 @@ class GradientTape(object):
  self._persistent = persistent
  self._watch_accessed_variables = watch_accessed_variables
  self._recording = False
  + self._created_eagerly = context.executing_eagerly()
  + if self._created_eagerly:
  context.context().start_step()
  def __enter__(self):
  @@ -755,6 +757,7 @@ class GradientTape(object):
  self._recording = False
  def __del__(self):
  + if self._created_eagerly:
  context.context().end_step()
  def watch(self, tensor):
  