How to change the objective function?

Dear Dr. Hai Shu,

I would like to use your metric in my future projects.
But I cannot figure out where the objective function (e.g. Cross Entropy) is implemented in the codes. It seems to me that the objective function is already implemented in grad_f objects [as nabla_f(omega0) regarding your original article]. Can you help me clear up my confusion?

Thank you very much 

Best regards