Add interface for sequential models

[mcwitt]

Once #477 or similar is merged, we could provide an API similar to pytorch's Sequential using for example a Sequential kind wrapping a type-level list, e.g.

data Sequential (models :: [k]) = Sequential (HList models)

and a HasForward instances like

instance HasForward (Sequential '[]) input where
  type Output (Sequential '[]) input = input
  forward _ input = input

instance
  (HasForward model input, HasForward (Sequential models) (Output model input)) =>
  HasForward (Sequential (model ': models)) input
  where
  type Output (Sequential (model ': models)) input = Output (Sequential models) (Output model input)
  forward (Sequential (model :. models)) input = forward (Sequential models) . forward model $ input

(credit for the idea and sketch implementation goes to @tscholak)