Add prompt scheduling callback to community scripts

Author: hlky

examples/community/README_community_scripts.md

@@ -8,6 +8,7 @@ If a community script doesn't work as expected, please open an issue and ping th
 |:--------------------------------------------------------------------------------------------------------------------------------------|:---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|:------------------------------------------------------------------------------------------|:-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|--------------------------------------------------------------:|
 | Using IP-Adapter with negative noise                                                                                                  | Using negative noise with IP-adapter to better control the generation (see the [original post](https://github.com/huggingface/diffusers/discussions/7167) on the forum for more details)                                                                                                                                                                                                                                                    | [IP-Adapter Negative Noise](#ip-adapter-negative-noise)                                   | | [Álvaro Somoza](https://github.com/asomoza)|
 | asymmetric tiling                                                                                                  |configure seamless image tiling independently for the X and Y axes                                                                                                                                                                                                      | [Asymmetric Tiling](#asymmetric-tiling )                                   | | [alexisrolland](https://github.com/alexisrolland)|
+| Prompt scheduling callback                                                                                                  |Allows changing prompts during a generation                                                                                                                                                                                                      | [Prompt Scheduling](#prompt-scheduling )                                   | | [hlky](https://github.com/hlky)|
 
 
 ## Example usages
@@ -229,4 +230,86 @@ seamless_tiling(pipeline=pipeline, x_axis=False, y_axis=False)
 
 torch.cuda.empty_cache()
 image.save('image.png')
-```
+```
+
+### Prompt Scheduling callback
+
+Prompt scheduling callback allows changing prompts during a generation, like [prompt editing in A1111](https://github.com/AUTOMATIC1111/stable-diffusion-webui/wiki/Features#prompt-editing)
+
+```python
+from diffusers import StableDiffusionPipeline
+from diffusers.callbacks import PipelineCallback, MultiPipelineCallbacks
+from diffusers.configuration_utils import register_to_config
+import torch
+from typing import Any, Dict, Optional
+
+
+pipeline: StableDiffusionPipeline = StableDiffusionPipeline.from_pretrained(
+    "stable-diffusion-v1-5/stable-diffusion-v1-5",
+    torch_dtype=torch.float16,
+    variant="fp16",
+    use_safetensors=True,
+).to("cuda")
+pipeline.safety_checker = None
+pipeline.requires_safety_checker = False
+
+
+class SDPromptScheduleCallback(PipelineCallback):
+    @register_to_config
+    def __init__(
+        self,
+        prompt: str,
+        negative_prompt: Optional[str] = None,
+        num_images_per_prompt: int = 1,
+        cutoff_step_ratio=1.0,
+        cutoff_step_index=None,
+    ):
+        super().__init__(
+            cutoff_step_ratio=cutoff_step_ratio, cutoff_step_index=cutoff_step_index
+        )
+
+    tensor_inputs = ["prompt_embeds"]
+
+    def callback_fn(
+        self, pipeline, step_index, timestep, callback_kwargs
+    ) -> Dict[str, Any]:
+        cutoff_step_ratio = self.config.cutoff_step_ratio
+        cutoff_step_index = self.config.cutoff_step_index
+
+        # Use cutoff_step_index if it's not None, otherwise use cutoff_step_ratio
+        cutoff_step = (
+            cutoff_step_index
+            if cutoff_step_index is not None
+            else int(pipeline.num_timesteps * cutoff_step_ratio)
+        )
+
+        if step_index == cutoff_step:
+            prompt_embeds, negative_prompt_embeds = pipeline.encode_prompt(
+                prompt=self.config.prompt,
+                negative_prompt=self.config.negative_prompt,
+                device=pipeline._execution_device,
+                num_images_per_prompt=self.config.num_images_per_prompt,
+                do_classifier_free_guidance=pipeline.do_classifier_free_guidance,
+            )
+            if pipeline.do_classifier_free_guidance:
+                prompt_embeds = torch.cat([negative_prompt_embeds, prompt_embeds])
+            callback_kwargs[self.tensor_inputs[0]] = prompt_embeds
+        return callback_kwargs
+
+callback = MultiPipelineCallbacks(
+    [
+        SDPromptScheduleCallback(
+            prompt="Official portrait of a smiling world war ii general, female, cheerful, happy, detailed face, 20th century, highly detailed, cinematic lighting, digital art painting by Greg Rutkowski",
+            negative_prompt="Deformed, ugly, bad anatomy",
+            cutoff_step_ratio=0.25,
+        )
+    ]
+)
+
+image = pipeline(
+    prompt="Official portrait of a smiling world war ii general, male, cheerful, happy, detailed face, 20th century, highly detailed, cinematic lighting, digital art painting by Greg Rutkowski",
+    negative_prompt="Deformed, ugly, bad anatomy",
+    callback_on_step_end=callback,
+    callback_on_step_end_tensor_inputs=["prompt_embeds"],
+).images[0]
+```