Add Symbol.toStringTag for each class

[lionkunonly]

This PR can fix all the failed cases caused by using String to check class. The sample looks like:

  it('readNetwork should return a Network object', async () => {
    expect(await core.readNetwork(model_path, weights_path)).to.be.a('Network');
  });

This case is failed before the PR. And the PR fixes all similar failed test cases.