class_docstr: Naive model with a simplest structure for testing purposes.
  
  Bare minimum functioning model. The best choice to get things rolling.
  The worst choice to fit and evaluate performance.
docstr: Build.
docstr_tok: ["Build","."]
code: def build(self):
      """Build."""
      x_in = self._make_inputs()
      x = keras.layers.concatenate(x_in)
      x_out = self._make_output_layer()(x)
      self._backend = keras.models.Model(inputs=x_in, outputs=x_out)
  
imports: import keras
  from matchzoo.engine.base_model import BaseModel
  from matchzoo.engine import hyper_spaces
label_desc: Use image node_id 2 for calling the Naive obj's underlying member method code with example usage: obj.build() without return types