Make layer plugin directly work with ml api.

[zhoonit]

layer plugin developer has some burden to create and use the layer right away.

let's assume there is a custom layer

class MyLayer : public nntrainer::Layer {
/** define interfaces */
};

With this, below code is not only invalid, but will compile and segfault, because we have defined (implicitly) that only layerNode is a valid layer. not nntrainer::Layer.

int main() {
  auto model =  ml::train::createModel();
  auto my_layer = std::make_shared<ml::train::Layer>(new MyLayer);

  model->addLayer(my_layer);
}

option 1. we make above code valid. ( = make nntrainer::layerNode not type of ml::train::Layer)

Pass only layer not layerNode between codes while layerNode is only available inside NeuralNetworks

option 2. we make below code valid. ( = make nntrainer::Layer not type of ml::train::Layer)

int main() {
  auto model =  ml::train::createModel();
  auto my_layer = std::make_shared<ml::train::Layer>(new MyLayer);
  auto ML_API_layer = ml::train::createCustomLayer(my_layer);
  model->addLayer(ML_API_layer);
}

opinion

option 2 will need api change, option 1. need massive changes, although I think option 1. is more appropriate.

[taos-ci]

:octocat: cibot: Thank you for posting issue #1751. The person in charge will reply soon.

[zhoonit]

It turned out that we are already at option 2. but we lack of API to create LayerNode from plain layer, we will need to support this.