It is really not supported/documented for end users.
You could check if this tutorial is still valid:

  
      

      Medium – 10 Jun 19
  

  
    

Creating a TensorFlow CNN in C++ (Part 2) 34

  In this post I’ll show how to create, train and test a Convolutional Neural Network using the TensorFlow C++ API

  
    Reading time: 13 min read
  

  

  
    
    
  

  


More in general, what is your use case? Why do you need to train the model in c++?