Visualization of training in GUI

[Michelvl92]

I would love to add simple training visualization like in the torch-ngp repo.

They fixed this nicely in a small loop here: https://github.com/ashawkey/torch-ngp/blob/ccc703001720119e6b926c6d6971e0433c492b89/nerf/gui.py#L429

I would have fixed this in a PyTorch Lightning callback, e.g. by calling a test-step on train-step end. But I think this is impossible since dpg.is_dearpygui_running() should be called infidelity in a while look such as shown on L429 of the torch-ngp repo.

For the initial version, I only want to show the image, without any training settings, which is not needed at the moment. Could you help me (by explaining conceptually or by pseudo-code) how I could implement this? I would love to make a PR.