Return a layer output.
    :param x: tensor, the input to the network.
    :param layer: str, the name of the layer to compute.
    :param **kwargs: dict, extra optional params to pass to self.fprop.
    :return: the content of layer `layer`

def get_layer(self, x, layer, **kwargs):
    """Return a layer output.
    :param x: tensor, the input to the network.
    :param layer: str, the name of the layer to compute.
    :param **kwargs: dict, extra optional params to pass to self.fprop.
    :return: the content of layer `layer`
    """
    return self.fprop(x, **kwargs)[layer]