MjSceneParser.create_scene doesn't clean up properly

[zchuning]

In MjSceneParser.create_scene, the all_objects variable is a itertools.chain object which is exhausted after the first iteration. This means self.cleanup(all_objects) always receives an empty iterator. This results in a new temporary robot xml file being created under d3il_sim/model/mj/robots/ each time an environment is instantiated. Below is a proposed fix.

def create_scene(self, mj_robots, mj_surrounding, object_list: list, dt):
    if object_list is None:
        object_list = []
    if not isinstance(object_list, list):
        object_list = [object_list]

    # Build a single list so we can iterate multiple times
    all_objects = chain([mj_surrounding], object_list, mj_robots)
    all_objects = [obj for obj in all_objects]

    for obj in all_objects:
        self.load_mj_loadable(obj)

    self.set_dt(dt)

    model = mujoco.MjModel.from_xml_string(self.mj_xml_string, self.assets)
    data = mujoco.MjData(model)

    # all_objects is a real list now, not an exhausted iterator
    self.cleanup(all_objects)
    return model, data