Construct a max pooling layer.

def mpool(self,
              k_height,
              k_width,
              d_height=2,
              d_width=2,
              mode="VALID",
              input_layer=None,
              num_channels_in=None):
        """Construct a max pooling layer."""
        return self._pool("mpool", pooling_layers.max_pooling2d, k_height,
                          k_width, d_height, d_width, mode, input_layer,
                          num_channels_in)