Add mid-kdiffusion cfgdenoiser script callback - access latents, conditionings and sigmas mid-sampling

[dfaker]

Briefly discussed in the context allowing scripts to force various symmetries onto the image latents here: #2441

Script with basic mirroring and rotation operation here:
https://gist.github.com/dfaker/ac031e87174a94d8a170d897caac9ff6

[AUTOMATIC1111]

This kind of looks like if the user assigns something to those params in the callback, it won't have any effect. You either have to use fields from params in the code below, or re-assign local variables:

params = CGFDenoiserParams(x_in, image_cond_in, sigma_in, state.sampling_step, state.sampling_steps)
cfg_denoiser_callback(params)
x_in = params.x_in
...

Also let's not not name those _in and you should write description for what those fields do in params.

[dfaker]

Fair enough, I was imagining mutation of those properties in place as the useful ones are all objects, but assigning back now.