Automatic backends uninstalling proposal

[Pinucci0]

I propose a function for better management of the uninstallation of LocalAI models and their respective backends. Currently, when an AI model is installed, its corresponding backend is automatically installed as well. I propose that when the last model using a specific backend is uninstalled (i.e., no other model requires that backend), LocalAI should prompt the user to also uninstall the respective backend for cleanup in the gallery and on disk.