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Joint Fine-tuning and Conversion of Pretrained Speech and Language Models towards Linear Complexity
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Proposes CALD, which jointly converts a pretrained transformer (RoBERTa, Pythia, Wav2Vec2) into a linear-complexity model (Linformer, Mamba, Mamba2) and fine-tunes it on the target task via layerwise distillation with several guidance modes.
Problem
Linear-complexity transformer substitutes (Linformer, Mamba, Mamba2) usually lack pretrained checkpoints, especially for speech, and re-pretraining them is costly; converting existing pretrained transformers during fine-tuning is a cheaper alternative.
Approach
Cross-Architecture Layerwise Distillation: initialize the student with transferred non-attention parameters and distill hidden states after the feed-forward layer from a target teacher, with extra guidance modes (trajectory-guided teacher updates, waypoint checkpoints, and a hybrid of guided then unguided training) to retain pretrained knowledge.
Key results
- On GLUE-style tasks converting RoBERTa to Linformer, trajectory-guided CALD reaches 92.5% average accuracy (+0.2% over the re-pretrained Linformer at 92.3%), while unguided conversion drops to 80.0%.
- For Wav2Vec2 to Mamba2 speech conversion, target-guided CALD gets 6.56% WER on TED-LIUM vs 6.24% for standard Wav2Vec2 and 11.29% unguided; hybrid reaches 6.41% WER and 91.23% SLURP IC accuracy.
- On Pythia-1B to Mamba conversion with 2% Pile (6.0B tokens), hybrid CALD reaches 0.525 average zero-shot accuracy vs 0.514 unguided and 0.544 for the original Pythia-1B.
Measured effects reported in the paper
| What was changed | Where | Measured | Effect |
|---|---|---|---|
| Add target-guided layerwise distillation | RoBERTa-to-Linformer conversion · QNLI, QQP, SST2, IMDB (Average) | accuracy: 80.0% -> 91.6% | Helped |
| Add trajectory-guided distillation instead of target-guided distillation (ablation) | RoBERTa-to-Linformer conversion · QNLI, QQP, SST2, IMDB (Average) | accuracy: 91.6% -> 92.5% | Helped |
| Use waypoint-guided distillation instead of target-guided distillation (ablation) | RoBERTa-to-Linformer conversion · QNLI, QQP, SST2, IMDB (Average) | accuracy: 91.6% -> 92.1% | Helped |
| Switch from target-guided to unguided training (ablation) | RoBERTa-to-Linformer conversion · QNLI, QQP, SST2, IMDB (Average) | accuracy: 91.6% -> 89.9% | Hurt |
| Initialize target-guided conversion from source teacher parameters rather than target teacher parameters (ablation) | RoBERTa-to-Linformer conversion · QNLI, QQP, SST2, IMDB (Average) | accuracy: 91.6% -> 91.4% | Hurt |
| Add target-guided distillation | Pythia-1B-to-Mamba conversion · Zero-shot downstream tasks, 0.5% Pile / 1.5B tokens (Avg.) | score: 0.479 -> 0.495 | Helped |
| Use hybrid guidance instead of target guidance (ablation) | Pythia-1B-to-Mamba conversion · Zero-shot downstream tasks, 1% Pile / 3.0B tokens (Avg.) | score: 0.507 -> 0.519 | Helped |
| Add target-guided distillation | Wav2Vec2-to-Mamba2 conversion · TED-LIUMv3 test | WER: 11.29 -> 6.56 | Helped |
| Add target-guided distillation | Wav2Vec2-to-Mamba2 conversion · SLURP | intent classification accuracy: 79.68 -> 90.43 | Helped |
| Add target-guided distillation | Wav2Vec2-to-Mamba2 conversion · VoxCeleb1 | speaker ID accuracy: 84.24 -> 96.56 | Helped |
| Use waypoint-guided rather than target-guided distillation (ablation) | Wav2Vec2-to-Mamba2 conversion · TED-LIUMv3, SLURP, VoxCeleb1 | WER / accuracy: 6.56 -> 6.92; 90.43 -> 90.32; 96.56 -> 96.16 | Mixed |
| Switch from target-guided to unguided training (ablation) | Wav2Vec2-to-Mamba2 conversion · TED-LIUMv3, SLURP, VoxCeleb1 | WER / accuracy: 6.56 -> 6.41; 90.43 -> 91.23; 96.56 -> 96.41 | Mixed |
Surprising findings
- Waypoint-guided distillation was not helpful or even harmful for speech tasks, because Wav2Vec2 hidden states shift strongly early in fine-tuning (cosine distance 0.372 after 2,000 steps), unlike RoBERTa.
- Output (KD) distillation led to suboptimal results in language processing and language modeling, so the KD weight was set to zero there.
Limitations
- Language modeling experiments are limited to Pythia-1B on a tiny subset of the Pile due to compute, and show a remaining gap to the original model.
- Unguided training is unstable and fails to converge for large Mamba2 speech models trained from scratch or with transferred parameters.
- Inference speed comparison is limited; the actual speedup depends on implementation and was not investigated in depth.
Experimental setup
Datasets: QNLI QQP SST2 IMDB Pile TED-LIUMv3 SLURP VoxCeleb1
Benchmarks: QNLI QQP SST2 IMDB Lambada PIQA Winogrande WSC ARC-Easy ARC-Challenge SciQ LogiQA
Metrics: accuracy word error rate zero-shot accuracy
Baselines: Unguided parameter transfer Re-pretrained Linformer Standard fine-tuned RoBERTa Standard fine-tuned Wav2Vec2 Pythia-1B
Builds on: Linformer Mamba Mamba2 knowledge distillation Wav2Vec2
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