Takes out index from the parameter's tensor name. E.g. parameter tensor name is 
        GPR-0000/kern/lengthscales, the method for that parameter will return '0000' index.

def tf_compilation_index(self):
        """
        Takes out index from the parameter's tensor name. E.g. parameter tensor name is 
        GPR-0000/kern/lengthscales, the method for that parameter will return '0000' index.
        """
        if self.parameter_tensor is None:
            return None
        name = self.parameter_tensor.name
        return name.split('-', 1)[-1].split('/')[0]