message: [Grappler] Add comment about the need to set feeds and fetches in signature_def when
  calling tf_optimizer.
diff: @@ -29,7 +29,11 @@ def OptimizeGraph(config_proto,
  verbose=True,
  graph_id=b'graph_to_optimize',
  cluster=None):
  - """Optimize the provided metagraph."""
  + """Optimize the provided metagraph.
  +
  + For best results, the signature_def field in `metagraph` should be populated
  + with information about input (feed) and output (fetch) tensors.
  + """
  if not isinstance(config_proto, config_pb2.ConfigProto):
  raise TypeError('Expected config_proto to be a ConfigProto, saw type %s' %
  type(config_proto))
  