message: Add a warning for dump_graph when dirname is not specified
diff: @@ -84,6 +84,10 @@ string WriteTextProtoToUniqueFile(Env* env, const string& name,
  dir = getenv("TF_DUMP_GRAPH_PREFIX");
  }
  if (!dir) {
  + LOG(WARNING)
  + << "Failed to dump " << name << " because dump location is not "
  + << " specified through either TF_DUMP_GRAPH_PREFIX environment "
  + << "variable or function argument.";
  return "(TF_DUMP_GRAPH_PREFIX not specified)";
  }
  Status status = env->RecursivelyCreateDir(dir);
  