fix: fix ValueError in NunchakuQwenImagePipeline.prepare_latents unpacking (#616)

* fix: enhance latent variable preparation in NunchakuQwenImagePipeline

- Refactored latent variable preparation to utilize the parent method for generating latents.
- Added manual generation of latent_image_ids to ensure correct indexing.

* refactor: clean up whitespace in NunchakuQwenImagePipeline

- Removed unnecessary whitespace in the latent variable preparation section for improved readability.

nunchaku/pipeline/pipeline_qwenimage.py

@@ -96,7 +96,9 @@ class NunchakuQwenImagePipeline(QwenImagePipeline):
 
         # 4. Prepare latent variables
         num_channels_latents = self.transformer.config.in_channels // 4
-        latents, latent_image_ids = self.prepare_latents(
+
+        # Get latents from parent method (returns single tensor)
+        latents = super().prepare_latents(
             batch_size * num_images_per_prompt,
             num_channels_latents,
             height,
@@ -106,6 +108,12 @@ class NunchakuQwenImagePipeline(QwenImagePipeline):
             generator,
             latents,
         )
+
+        # Generate latent_image_ids manually
+        seq_len = latents.shape[1] if len(latents.shape) > 1 else 0
+        latent_image_ids = torch.arange(seq_len, device=device, dtype=torch.long)
+        latent_image_ids = latent_image_ids.unsqueeze(0).repeat(batch_size * num_images_per_prompt, 1)
+
         img_shapes = [(1, height // self.vae_scale_factor // 2, width // self.vae_scale_factor // 2)] * batch_size
 
         # 5. Prepare timesteps