feat(nodes): add list_images endpoint

- add `list_images` endpoint at `GET api/v1/images`
- extend `ImageStorageBase` with `list()` method, implemented it for `DiskImageStorage`
- add `ImageReponse` class to for image responses, which includes urls, metadata
- add `ImageMetadata` class (basically a stub at the moment)
- uploaded images now named `"{uuid}_{timestamp}.png"`
- add `models` modules. besides separating concerns more clearly, this helps to mitigate circular dependencies
- improve thumbnail handling

invokeai/app/util/step_callback.py

@@ -0,0 +1,43 @@
+import torch
+from ..invocations.baseinvocation import InvocationContext
+from ...backend.util.util import image_to_dataURL
+from ...backend.generator.base import Generator
+from ...backend.stable_diffusion import PipelineIntermediateState
+
+def fast_latents_step_callback(
+    sample: torch.Tensor,
+    step: int,
+    steps: int,
+    id: str,
+    context: InvocationContext,
+):
+    # TODO: only output a preview image when requested
+    image = Generator.sample_to_lowres_estimated_image(sample)
+
+    (width, height) = image.size
+    width *= 8
+    height *= 8
+
+    dataURL = image_to_dataURL(image, image_format="JPEG")
+
+    context.services.events.emit_generator_progress(
+        context.graph_execution_state_id,
+        id,
+        {"width": width, "height": height, "dataURL": dataURL},
+        step,
+        steps,
+    )
+
+
+def diffusers_step_callback_adapter(*cb_args, **kwargs):
+    """
+    txt2img gives us a Tensor in the step_callbak, while img2img gives us a PipelineIntermediateState.
+    This adapter grabs the needed data and passes it along to the callback function.
+    """
+    if isinstance(cb_args[0], PipelineIntermediateState):
+        progress_state: PipelineIntermediateState = cb_args[0]
+        return fast_latents_step_callback(
+            progress_state.latents, progress_state.step, **kwargs
+        )
+    else:
+        return fast_latents_step_callback(*cb_args, **kwargs)