Remove summaries from the default graph.

def remove_summaries():
  """Remove summaries from the default graph."""
  g = tf.get_default_graph()
  key = tf.GraphKeys.SUMMARIES
  log_debug("Remove summaries %s" % str(g.get_collection(key)))
  del g.get_collection_ref(key)[:]
  assert not g.get_collection(key)