docs: def _build(self, inputs, prev_state):
      """Connects the highway core module into the graph.
  
      Args:
        inputs: Tensor of size `[batch_size, input_size]`.
        prev_state: Tensor of size `[batch_size, hidden_size]`.
  
      Returns:
        A tuple (output, next_state) where `output` is a Tensor of size
        `[batch_size, hidden_size]` and `next_state` is a Tensor of size
        `[batch_size, hidden_size]`.
  
      Raises:
        ValueError: If connecting the module into the graph any time after the
          first time, and the inferred size of the inputs does not match previous
          invocations.
      """
      input_size = inputs.get_shape()[1]
      weight_shape = (input_size, self._hidden_size)
      u_shape = (self._hidden_size, self._hidden_size)
      bias_shape = (self._hidden_size,)
  
      def _get_variable(name, shape):
        return tf.get_variable(
            name,
            shape,
            dtype=inputs.dtype,
            initializer=self._initializers.get(name),
            partitioner=self._partitioners.get(name),
            regularizer=self._regularizers.get(name))
  
      pre_highway_wt = _get_variable(self.WT, weight_shape)
      pre_highway_wh = _get_variable(self.WH, weight_shape)
      state = prev_state
      for layer_index in xrange(self._num_layers):
        layer_str = str(layer_index)
        layer_wt = _get_variable(self.WT + layer_str, u_shape)
        layer_bt = _get_variable(self.BT + layer_str, bias_shape)
        layer_wh = _get_variable(self.WH + layer_str, u_shape)
        layer_bh = _get_variable(self.BH + layer_str, bias_shape)
        linear_t = tf.matmul(state, layer_wt) + layer_bt
        linear_h = tf.matmul(state, layer_wh) + layer_bh
        if layer_index == 0:
          linear_t += tf.matmul(inputs, pre_highway_wt)
          linear_h += tf.matmul(inputs, pre_highway_wh)
        output_t = tf.sigmoid(linear_t)
        output_h = tf.tanh(linear_h)
        state = state * (1 - output_t) + output_h * output_t
  
      return state, state