fix(qwen-image dreambooth): correct prompt embed repeats when using --with_prior_preservation (#13396)

fix(qwen): correct prompt embed repeats with prior preservation

Co-authored-by: Sayak Paul <spsayakpaul@gmail.com>

Author: chenyangzhu1

examples/dreambooth/train_dreambooth_lora_qwen_image.py

@@ -1529,7 +1529,10 @@ def get_sigmas(timesteps, n_dim=4, dtype=torch.float32):
                     prompt_embeds = prompt_embeds_cache[step]
                     prompt_embeds_mask = prompt_embeds_mask_cache[step]
                 else:
-                    num_repeat_elements = len(prompts)
+                    # With prior preservation, prompt_embeds already contains [instance, class] embeddings
+                    # from the cat above, but collate_fn also doubles the prompts list. Use half the
+                    # prompts count to avoid a 2x over-repeat that produces more embeddings than latents.
+                    num_repeat_elements = len(prompts) // 2 if args.with_prior_preservation else len(prompts)
                     prompt_embeds = prompt_embeds.repeat(num_repeat_elements, 1, 1)
                     if prompt_embeds_mask is not None:
                         prompt_embeds_mask = prompt_embeds_mask.repeat(num_repeat_elements, 1)