Don't load model on startup

[ljleb]

Download the LCM model during the first generation, not during script loading. from_pretrained should reuse an already downloaded model.

This makes manual LDM unloading unnecessary.

Related: #12 points 2 and 3

[ljleb]

Notes:

• now the LCM is downloaded during the first click on run
• any LDM loaded prior to LCM inference is unloaded from memory before loading the LCM
• when reloading a LDM model, the LCM is automatically unloaded from memory before loading the LDM
• if the user clicks on the LCM "Run" button too soon after opening a browser on the webui, any networks that remain to be discovered will raise an exception because the procedure that does this uses properties of the LDM object. For this purpose, the "Run" button is disabled and then enabled later whenever all lora networks have been consolidated into the model cards ui. This is a hack, and will not be necessary as soon as we have proper model integration in the checkpoints dropdown