Diffusion trainer fix: shift logits to align with input tokens (#3191)

* shift logits for diffusion generate

* delete unused

* diffusion trainer: token shift

src/axolotl/integrations/diffusion/generation.py

@@ -7,7 +7,7 @@ import torch
 
 from axolotl.utils.logging import get_logger
 
-from .utils import create_bidirectional_attention_mask
+from .utils import create_bidirectional_attention_mask, shift_logits_to_input_positions
 
 LOG = get_logger(__name__)
 
@@ -360,7 +360,7 @@ def _diffusion_step(
 
     # Forward pass
     outputs = model(input_ids=sequence, attention_mask=attention_mask)
-    logits = outputs.logits
+    logits = shift_logits_to_input_positions(outputs.logits)
 
     # Only sample at currently masked positions
     if current_mask.any():

src/axolotl/integrations/diffusion/trainer.py

@@ -11,7 +11,7 @@ from axolotl.utils.dict import DictDefault
 from axolotl.utils.logging import get_logger
 
 from .callbacks import DiffusionGenerationCallback
-from .utils import create_bidirectional_attention_mask
+from .utils import create_bidirectional_attention_mask, shift_logits_to_input_positions
 
 LOG = get_logger(__name__)
 
@@ -207,7 +207,7 @@ class DiffusionTrainer(AxolotlTrainer):
             input_ids=noisy_batch.long(),
             attention_mask=bidirectional_mask,
         )
-        logits = outputs.logits
+        logits = shift_logits_to_input_positions(outputs.logits)
 
         if masked_indices.sum() > 0:
             valid_indices = torch.where(masked_indices)

src/axolotl/integrations/diffusion/utils.py

@@ -157,3 +157,10 @@ def create_bidirectional_attention_mask(
 
     # Add head dimension: [batch_size, 1, seq_len, seq_len]
     return bidirectional_mask.unsqueeze(1)
+
+
+def shift_logits_to_input_positions(logits: torch.Tensor) -> torch.Tensor:
+    """Align next-token logits with their input token positions for diffusion."""
+    if logits.size(1) <= 1:
+        return logits
+    return torch.cat([logits[:, :1], logits[:, :-1]], dim=1)