code: def chain(*layers):
      
      if len(layers) == 0:
          return FeedForward([])
      elif len(layers) == 1:
          return layers[0]
      else:
          return FeedForward(layers)
docs: Compose two models `f` and `g` such that they become layers of a single
      feed-forward model that computes `g(f(x))`.
  
      Raises exception if their dimensions don't match.