Type hint for `callback_on_step_end` in pipeline `__call__` is incorrect

[philpax]

Describe the bug

As far as I can tell, the type hint callback_on_step_end for all pipelines (as seen in this search) is incorrect.

Taking the SDXL pipeline as an example, the type hint in __call__ is:

callback_on_step_end: Optional[Callable[[int, int, Dict], None]] = None

but it is called like this:

if callback_on_step_end is not None:
    callback_kwargs = {}
    for k in callback_on_step_end_tensor_inputs:
        callback_kwargs[k] = locals()[k]
    callback_outputs = callback_on_step_end(self, i, t, callback_kwargs)

    latents = callback_outputs.pop("latents", latents)
    prompt_embeds = callback_outputs.pop("prompt_embeds", prompt_embeds)
    negative_prompt_embeds = callback_outputs.pop("negative_prompt_embeds", negative_prompt_embeds)
    add_text_embeds = callback_outputs.pop("add_text_embeds", add_text_embeds)
    negative_pooled_prompt_embeds = callback_outputs.pop(
        "negative_pooled_prompt_embeds", negative_pooled_prompt_embeds
    )
    add_time_ids = callback_outputs.pop("add_time_ids", add_time_ids)
    negative_add_time_ids = callback_outputs.pop("negative_add_time_ids", negative_add_time_ids)

That is, the type hint suggests that it's a function with three arguments that returns nothing, but it's actually a function with four arguments that returns a Dict. This ends up failing at runtime. Using a four-argument function leads to type-checker errors.

Reproduction

Using a Python typechecker (Pyright in my case), attempt to use a correctly-defined callback with a SDXL pipeline:

sdxl_pipe = AutoPipelineForText2Image.from_pretrained(
    "stabilityai/sdxl-turbo", torch_dtype=torch.float16, variant="fp16"
)
sdxl_pipe = typing.cast(StableDiffusionXLPipeline, sdxl_pipe)

def callback_on_step_end(_pipe, step, _timestep, kwargs):
    print(step)
    return kwargs

output = sdxl_pipe(
    prompt="a prompt",
    negative_prompt=self.negative_prompt,
    num_inference_steps=4,
    guidance_scale=0.0,
    return_dict=True,
    width=512,
    height=512,
    callback_on_step_end=callback_on_step_end,
)

The type-checker will error on callback_on_step_end:

Argument of type "(_pipe: Unknown, step: Unknown, _timestep: Unknown, kwargs: Unknown) -> Unknown" cannot be assigned to parameter "callback_on_step_end" of type "((int, int, Dict[Unknown, Unknown]) -> None) | None" in function "__call__"
  Type "(_pipe: Unknown, step: Unknown, _timestep: Unknown, kwargs: Unknown) -> Unknown" cannot be assigned to type "((int, int, Dict[Unknown, Unknown]) -> None) | None"
    Type "(_pipe: Unknown, step: Unknown, _timestep: Unknown, kwargs: Unknown) -> Unknown" cannot be assigned to type "(int, int, Dict[Unknown, Unknown]) -> None"
      Function accepts too few positional parameters; expected 4 but received 3
    "function" is incompatible with "None"

This can be worked around using # type: ignore, which is what I'm doing.

Logs

No response

System Info

N/A

Who can help?

No response

[yiyixuxu]

Thanks for creating this issue do you want to open a PR to fix the type hint?

[philpax]

Hi, sorry for the late response! Yes, I can take a look at it; should hopefully be a relatively straightforward fix.

Edit: I'm not currently working with diffusers, so I haven't been able to work on this fix. Free for anyone else to take it.

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