adjust the API of KL methods for model selection

make the API work for `GradientModelSelection`
@karlnapf 
please merge this.
The job of model selection is to optimize `hyper-parameters`  of a model.
In Shogun, it is done via `m_gradient_parameters`.

However, `variational parameters` and `dual parameter(s)` are NOT  `hyper-parameters`  of a model. These `variational parameters` and `dual parameter(s)` are optimized during inference process. In order words, they are latent variables.
This also explains the below question.