Parametric Loss Objects

[aytekinar]

I am not sure if we will get rid of all the Loss representations to favor packages like LossFunctions.jl, but we can consider rewriting some common loss objects as parametric types. Check below:

struct LogLoss{Index<:Integer, Value<:AbstractFloat} <: AbstractLoss
  # ...
end

I will make a stab now on a different branch for my experiments. We can discuss later.