How to use the fontmake.instantiator function in fontmake

def test_axis_mapping(data_dir):
    designspace = designspaceLib.DesignSpaceDocument.fromfile(
        data_dir / "DesignspaceTest" / "DesignspaceTest-wght-wdth.designspace"
    )
    generator = fontmake.instantiator.Instantiator.from_designspace(
        designspace, round_geometry=True
    )

    instance_font = generator.generate_instance(designspace.instances[0])
    assert instance_font.info.openTypeOS2WeightClass == 400
    assert instance_font.info.openTypeOS2WidthClass == 5
    assert instance_font.info.italicAngle is None
    assert instance_font.lib["designspace.location"] == [
        ("weight", 100.0),
        ("width", 100.0),
    ]