[Feature]: Control: if a ControlNet model is used in multiple units, load it only once

[lbeltrame]

Feature description

With the emergence of ControlNet Union, the same model may be used in more than one unit (in multi controlnets). As of now, Control loads the model every time there is a unit requesting it: for example, for 3 units it would get loaded three times. That's pretty wasteful of VRAM if the model is just one, therefore it may be desirable to load it once, regardless of how many units are enabled.

As I wrote above, this feature may have a valid use-case now that we have CN Union as it can be used more than once with different input images.

Version Platform Description

Latest dev as of 2024-07-10'.