make step_callback work again in generate() call

[lstein]

This PR fixes #2951 and restores the step_callback argument in the refactored generate() method. Note that this issue states that "something is still wrong because steps and step are zero." However, I think this is confusion over the call signature of the callback, which since the diffusers merge has been callback(state:PipelineIntermediateState)

This is the test script that I used to determine that step is being passed correctly:

from pathlib import Path
from invokeai.backend import ModelManager
from invokeai.backend.globals import global_config_dir
from invokeai.backend.generator import Txt2Img
from invokeai.backend.stable_diffusion import PipelineIntermediateState

def my_callback(state:PipelineIntermediateState, total_steps:int):
    print(f'callback(step={state.step}/{total_steps})')

def main():
    manager = ModelManager(Path(global_config_dir()) / "models.yaml")
    model = manager.get_model('stable-diffusion-1.5')
    print ('=== TXT2IMG TEST ===')
    steps=30
    output = next(Txt2Img(model).generate(prompt='banana sushi',
                                          iterations=None,
                                          steps=steps,
                                          step_callback=lambda x: my_callback(x,steps)
                                          )
                  )
    print(f'image={output.image}, seed={output.seed}, steps={output.params.steps}')

if __name__=='__main__':
    main()

[psychedelicious]

@Kyle0654 the signature of step_callback has changed so that will need to be updated on nodes. See example code from Lincoln

[psychedelicious]

thanks, state.step appears to start at -1. what does this mean?

[lstein]

@Kyle0654 the signature of step_callback has changed so that will need to be updated on nodes. See example code from Lincoln

The signature changed when we did the diffusers migration, so it actually a longstanding issue. I would have to defer to @keturn to explain why the first step is -1. It looks like the callback is called once after generation of the initial latent and before the first denoising step (which is step 0).