Multi Model running log call back support

[wandbrandon]

I am writing safe rust bindings for this package, and I want to allow the user of the library to have the ability to run two models concurrently. While spawning a thread for each model build and then running txt2img on each one is possible and works fine, the log callback function and the progress callback function are global and do not accept a model context. Therefore there is no way to know which logs and which steps are associated with each model context.

Logging should not be global, but should depend on the model context. Is this possible?

Thanks.