construct and return descriptor

def get_descriptor(ctx):
    """ construct and return descriptor """
    d_net = gluon.nn.Sequential()
    with d_net.name_scope():

        d_net.add(SNConv2D(num_filter=64, kernel_size=4, strides=2, padding=1, in_channels=3, ctx=ctx))
        d_net.add(gluon.nn.LeakyReLU(0.2))

        d_net.add(SNConv2D(num_filter=128, kernel_size=4, strides=2, padding=1, in_channels=64, ctx=ctx))
        d_net.add(gluon.nn.LeakyReLU(0.2))

        d_net.add(SNConv2D(num_filter=256, kernel_size=4, strides=2, padding=1, in_channels=128, ctx=ctx))
        d_net.add(gluon.nn.LeakyReLU(0.2))

        d_net.add(SNConv2D(num_filter=512, kernel_size=4, strides=2, padding=1, in_channels=256, ctx=ctx))
        d_net.add(gluon.nn.LeakyReLU(0.2))

        d_net.add(SNConv2D(num_filter=1, kernel_size=4, strides=1, padding=0, in_channels=512, ctx=ctx))

    return d_net