Exports signatures from current graph into a MetaGraphDef.

def _export_signatures(meta_graph):
  """Exports signatures from current graph into a MetaGraphDef."""
  named_signatures = tf_v1.get_collection(_SIGNATURE_COLLECTION)
  if not named_signatures:
    raise ValueError("No signatures present. Please call hub.add_signature(...)"
                     "at least once in the module_fn.")
  for key, signature in named_signatures:
    meta_graph.signature_def[key].CopyFrom(signature)