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[chunked loss] align teacher and student logit shape #634

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Merged
shivam15s merged 2 commits into from
Mar 28, 2025
Merged

[chunked loss] align teacher and student logit shape #634

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align teacher and student logit shape
yundai424 Mar 27, 2025
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yundai424 Mar 28, 2025
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15 changes: 15 additions & 0 deletions src/liger_kernel/chunked_loss/fused_linear_distillation.py
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Original file line number Diff line number Diff line change
Expand Up @@ -115,6 +115,21 @@ def _compute_loss(
student_logits_chunk /= temperature
teacher_logits_chunk /= temperature

# If the teacher and student token size is different, pad student logits to match the teacher's.
# This only applies to cases where they share exactly the same vocab and tokenizer just
# that teacher logit is padded for some training efficiency such as
# https://huggingface.co/Qwen/Qwen1.5-72B-Chat/discussions/1#662883f568adf59b07b176d2
teacher_vocab_size = teacher_weight.shape[0]
student_vocab_size = student_weight.shape[0]
if teacher_vocab_size > student_vocab_size:
pad_size = teacher_vocab_size - student_vocab_size
pad_tensor = torch.zeros(
(*student_logits_chunk.shape[:-1], pad_size),
dtype=student_logits_chunk.dtype,
device=student_logits_chunk.device,
)
student_logits_chunk = torch.cat([student_logits_chunk, pad_tensor], dim=-1)

hard_loss /= full_target.shape[0]

soft_loss = distillation_loss_fn(student_logits_chunk, teacher_logits_chunk, **loss_kwargs)
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