engine: ensure that all scopes are named (#1154)
engine: ensure that all scopes are named (#1154)

This was omitted in PR #1151, sorry for that. :disappointed: 

Refs: https://github.com/tensorflow/tfjs-core/pull/1151
tensorflow/tfjs,tensorflow/tfjs-core,tensorflow/tfjs-core,tensorflow/tfjs-core,tensorflow/tfjs-core,tensorflow/tfjs,tensorflow/tfjs-core,tensorflow/tfjs-core,tensorflow/tfjs,tensorflow/tfjs,tensorflow/tfjs,tensorflow/tfjs,tensorflow/tfjs,tensorflow/tfjs,tensorflow/tfjs