Handle prompt embedding concat in Qwen dreambooth example (#13387)

* Handle prompt embedding concat in Qwen dreambooth example

* remove wandb config

* Apply style fixes

* add a comment on how this is only relevant during prior preservation.

---------

Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>
Co-authored-by: Sayak Paul <spsayakpaul@gmail.com>

Author: 3 people

examples/dreambooth/train_dreambooth_lora_qwen_image.py

@@ -906,6 +906,68 @@ def __getitem__(self, index):
         return example
 
 
+# These helpers only matter for prior preservation, where instance and class prompt
+# embedding batches are concatenated and may not share the same mask/sequence length.
+def _materialize_prompt_embedding_mask(
+    prompt_embeds: torch.Tensor, prompt_embeds_mask: torch.Tensor | None
+) -> torch.Tensor:
+    """Return a dense mask tensor for a prompt embedding batch."""
+    batch_size, seq_len = prompt_embeds.shape[:2]
+
+    if prompt_embeds_mask is None:
+        return torch.ones((batch_size, seq_len), dtype=torch.long, device=prompt_embeds.device)
+
+    if prompt_embeds_mask.shape != (batch_size, seq_len):
+        raise ValueError(
+            f"`prompt_embeds_mask` shape {prompt_embeds_mask.shape} must match prompt embeddings shape "
+            f"({batch_size}, {seq_len})."
+        )
+
+    return prompt_embeds_mask.to(device=prompt_embeds.device)
+
+
+def _pad_prompt_embedding_pair(
+    prompt_embeds: torch.Tensor, prompt_embeds_mask: torch.Tensor | None, target_seq_len: int
+) -> tuple[torch.Tensor, torch.Tensor]:
+    """Pad one prompt embedding batch and its mask to a shared sequence length."""
+    prompt_embeds_mask = _materialize_prompt_embedding_mask(prompt_embeds, prompt_embeds_mask)
+    pad_width = target_seq_len - prompt_embeds.shape[1]
+
+    if pad_width <= 0:
+        return prompt_embeds, prompt_embeds_mask
+
+    prompt_embeds = torch.cat(
+        [prompt_embeds, prompt_embeds.new_zeros(prompt_embeds.shape[0], pad_width, prompt_embeds.shape[2])], dim=1
+    )
+    prompt_embeds_mask = torch.cat(
+        [prompt_embeds_mask, prompt_embeds_mask.new_zeros(prompt_embeds_mask.shape[0], pad_width)], dim=1
+    )
+
+    return prompt_embeds, prompt_embeds_mask
+
+
+def concat_prompt_embedding_batches(
+    *prompt_embedding_pairs: tuple[torch.Tensor, torch.Tensor | None],
+) -> tuple[torch.Tensor, torch.Tensor | None]:
+    """Concatenate prompt embedding batches while handling missing masks and length mismatches."""
+    if not prompt_embedding_pairs:
+        raise ValueError("At least one prompt embedding pair must be provided.")
+
+    target_seq_len = max(prompt_embeds.shape[1] for prompt_embeds, _ in prompt_embedding_pairs)
+    padded_pairs = [
+        _pad_prompt_embedding_pair(prompt_embeds, prompt_embeds_mask, target_seq_len)
+        for prompt_embeds, prompt_embeds_mask in prompt_embedding_pairs
+    ]
+
+    merged_prompt_embeds = torch.cat([prompt_embeds for prompt_embeds, _ in padded_pairs], dim=0)
+    merged_mask = torch.cat([prompt_embeds_mask for _, prompt_embeds_mask in padded_pairs], dim=0)
+
+    if merged_mask.all():
+        return merged_prompt_embeds, None
+
+    return merged_prompt_embeds, merged_mask
+
+
 def main(args):
     if args.report_to == "wandb" and args.hub_token is not None:
         raise ValueError(
@@ -1320,8 +1382,10 @@ def compute_text_embeddings(prompt, text_encoding_pipeline):
         prompt_embeds = instance_prompt_embeds
         prompt_embeds_mask = instance_prompt_embeds_mask
         if args.with_prior_preservation:
-            prompt_embeds = torch.cat([prompt_embeds, class_prompt_embeds], dim=0)
-            prompt_embeds_mask = torch.cat([prompt_embeds_mask, class_prompt_embeds_mask], dim=0)
+            prompt_embeds, prompt_embeds_mask = concat_prompt_embedding_batches(
+                (instance_prompt_embeds, instance_prompt_embeds_mask),
+                (class_prompt_embeds, class_prompt_embeds_mask),
+            )
 
     # if cache_latents is set to True, we encode images to latents and store them.
     # Similar to pre-encoding in the case of a single instance prompt, if custom prompts are provided