define shift_labels in gemma (#961)

## Summary
Minor fix: undefined variable in Gemma3/Paligemma
 
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## Details
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## Testing Done

- Hardware Type: 2x NVIDIA RTX 5880
- [x] run `make test` to ensure correctness
- [x] run `make checkstyle` to ensure code style
- [x] run `make test-convergence` to ensure convergence

Signed-off-by: Alexandros Koumparoulis <[email protected]>

Author: akoumpa

src/liger_kernel/transformers/model/gemma3.py

@@ -235,6 +235,7 @@ def multimodal_forward(
         **lm_kwargs,
     )
 
+    shift_labels = lm_kwargs.pop("shift_labels", None)
     hidden_states = outputs[0]
 
     slice_indices = slice(-logits_to_keep, None) if isinstance(logits_to_keep, int) else logits_to_keep

src/liger_kernel/transformers/model/paligemma.py

@@ -330,6 +330,7 @@ def lce_forward(
         **lm_kwargs,
     )
 
+    shift_labels = lm_kwargs.pop("shift_labels", None)
     hidden_states = outputs[0]
 
     loss = None