Expose a separate model XML for the simulator

[alberthli]

We should have the option to supply a different model XML for the simulator that's different from the XML that serves as the controller's internal model of the system. Then, we could do things like planning with a coarse step while simulating with a fine step without hacking the timestep manually in the scripts.

[vincekurtz]

This is already possible as follows:

task = MyTask()
ctrl = MyController(task, ...)

mj_model = mujoco.MjModel.from_xml_path("path/to/simulator/model.xml")
mj_data = mujoco.MjData(mj_model)
run_interactive(ctrl, mj_model, mj_data, ...)

In this case task.model is used by the controller, while mj_model is used by the simulator.

[alberthli]

Ah yes - to be clearer, I also think that for the subset of example tasks which motivated this issue, we should explicitly create such a new XML (e.g., with finer sim timesteps) to emphasize the differences between the sim and control models.