Created by: reyoung

Motivations

In the function package, class FunctionBase is the base class of Layer and Projection. Indeed, this design is not mandatory. We'd make replace class FunctionBase by std::function. An advantage of this change is that we can use Layers and Projections as they are C++ functions:

typedef std::function<Error(Tensors& inputs, Tensors& outputs)> CUDAKernel;
CUDAKernel softmax = KernelRegisters.get("softmax");
softmax(ins, outs);

Also, we can use std::bind to bind the value of some parameters, like we do with functions defined in std:

Error CosineKernel(Tensors& ins, Tensors& outs, double scale) {
  ...
}

KernelType cos = KernelRegister.get("cos", scale=3.0);
cos(ins, outs);

Actions Items

1. Change the use of classFunction in layer definitions intostd::function<Error(Tensors& inputs, Tensors& outputs)>:

   1. Decouple package function and layer.
   2. Define the conversion from FunctionBase to CUDAKernel.
   3. Above has been done in #2049.

2. Rewrite class Register in package function.