refactor NeuralNetworkApproximator with @forward (#221)

Author: norci

src/policies/q_based_policies/learners/approximators/neural_network_approximator.jl

@@ -2,6 +2,7 @@ export NeuralNetworkApproximator, ActorCritic
 
 using Flux
 import Functors: functor
+using MacroTools: @forward
 
 """
     NeuralNetworkApproximator(;kwargs)
@@ -21,12 +22,11 @@ end
 # some model may accept multiple inputs
 (app::NeuralNetworkApproximator)(args...; kwargs...) = app.model(args...; kwargs...)
 
+@forward NeuralNetworkApproximator.model Flux.testmode!, Flux.trainmode!, Flux.params, device
 
 functor(x::NeuralNetworkApproximator) =
     (model = x.model,), y -> NeuralNetworkApproximator(y.model, x.optimizer)
 
-device(app::NeuralNetworkApproximator) = device(app.model)
-
 RLBase.update!(app::NeuralNetworkApproximator, gs) =
     Flux.Optimise.update!(app.optimizer, params(app), gs)