message: Validate GraphDef after adding default attributes in GraphMgr
  The original ValidateGraphDef call will fail when we start stripping default
  attrs on the sender side. This CL turns on the same validation during
  GraphDef to Graph conversion, which happens after default attrs are added.
diff: @@ -135,11 +135,8 @@ Status GraphMgr::InitItem(const string& handle, const GraphDef& gdef,
  TF_RETURN_IF_ERROR(ValidateGraphDefForDevices(gdef));
  - if (gdef.versions().producer() >= 5) {
  - // Validate the graph: we assume that merging two valid graphs
  - // should maintain graph validity.
  - TF_RETURN_IF_ERROR(graph::ValidateGraphDef(gdef, *item->lib_def));
  - }
  + // We don't explicitly Validate the graph def because ConvertGraphDefToGraph
  + // does that below.
  item->proc_flr.reset(new ProcessFunctionLibraryRuntime(
  device_mgr_, worker_env_->env, gdef.versions().producer(),
  @@ -151,6 +148,7 @@ Status GraphMgr::InitItem(const string& handle, const GraphDef& gdef,
  GraphConstructorOptions opts;
  opts.allow_internal_ops = true;
  opts.expect_device_spec = true;
  + opts.validate_nodes = true;
  TF_RETURN_IF_ERROR(ConvertGraphDefToGraph(opts, gdef, &graph));
  // Splits "graph" into multiple subgraphs by device names.
  