Adding CorDA as an optional initialization method of LoRA

[iboing]

Existing PEFT methods are mostly agnoistic of the context of a task of concern, e.g., a downstraem task to learn or some pre-trained world knowledge to maintain. In our paper (https://openreview.net/pdf?id=Gi00NVru6n) accepted by NeurIPS 2024, we propose a PEFT method named Corda, context-oriented decomposition adaptation, which builds task-aware LoRA adapters from weight decomposition oriented by the context of the task concerned for task-aware PEFT.

Concretely, CorDA randomly collect a few (usually 256) data samples from a target task, e.g. questions from a QA dataset or instructions to write a code or solve a math problem, and feed these samples into a pre-trained LLM. We can obtain the covariance matrix of the input activation of each linear layer, i.e., $C=XX^T\in\mathcal{R}^{d_{in}\times d_{in}}$, where $X$ is the input of this linear layer. We then perform singular value decomposition (SVD) for the weight $W\in \mathcal{R}^{d_{out}\times d_{in}}$ multiplied by the covariance matrix, i.e., $\verb|SVD|(WC) = U\Sigma V^T$, where $U$ and $V$ are singular vectors and $\Sigma$ is the diagonal matrix with the singular values arranged in descending order. In this way, the context expressed by these representative covariance matrices is able to orientate the decomposition, such that the principal components are most associated with the task of concern. To ensure the same inference result with the pre-trained model at the start of adaptation, we multiply the inverse of these covariance matrices with the decomposed components, $\hat{W}=U\Sigma V^T C^{-1}$, where $\hat{W}$ is the weight after decomposition and reconstruction.

Thanks to the task-awareness, CorDA enables two optional modes, knowledge-preserving adaptation mode (KPM) and instruction-previewed adaptation mode (IPM). In KPM, we use questions from question-answering dataset whose knowledge needs to be preserved, such as TriviaQA and NQopen, to obtain the covariance matrices. After our context-oriented decomposition, we use the components with the smallest $r$ singular values, $U_{[:,-r:]}$, $\Sigma_{[-r:]}$, and $(V^TC^{-1})_{\left[-r:,:\right]}$ to initialize the learnable LoRA adapters $A=\sqrt{\Sigma}_{[-r:]}(V^TC^{-1})_{[-r:,:]}$ and $B=U_{[:,-r:]}\sqrt{\Sigma}_{[-r:]}$. The other components that compact the question-answering ability are frozen during adaptation. KPM enables to learn new tasks effectively while keeping the world knowledge associated with $C$ as sound as possible. Alternatively, when one only aims to achieve performance as high as possible on the finetuning task without concern for world knowledge maintenance, our IPM will be favored. In this mode, CorDA uses the instruction and response from the fine-tuning task (e.g., Math or Code) to produce the covariance matrices. The principal components with large singular values capturing the characteristics of the finetuning task in advance can better accommodate the new ability. So we initialize adapters as $A= \sqrt{\Sigma}_{[:r]} (V^T C^{-1})_{[:r,:]}$ and $B =U_{[:,:r]} \sqrt{\Sigma}_{[:r]}$, and freeze the remaining components. The implementations of KPM and IPM are compared as follows:

Mode	Collect covariance from	LoRA $A$	LoRA $B$
KPM	questions from a knowledge benchmark to maintain	$A=\sqrt{\Sigma}_{[-r:]}(V^T C^{-1})_{[-r:,:]}$	$B=U_{[:,-r:]}\sqrt{\Sigma}_{[-r:]}$
IPM	instructions and responses from a downstream task to learn	$A= \sqrt{\Sigma}_{[:r]} (V^T C^{-1})_{[:r,:]}$	$B =U_{[:,:r]} \sqrt{\Sigma}_{[:r]}$

Similar to PiSSA, our method is also an initialization method for LoRA adapter with the same LoRA structure, and our IPM also uses the principal components to initialize $A$ and $B$. But PiSSA adopts the naive SVD and does not consider task awareness. Our method builds task-aware adapters, enabling two optional modes to satisfy customized needs:

• KPM not only achieves better fine-tuning performance than LoRA but also effectively preserves pre-trained knowledge.
• IPM can further accelerate convergence and enhance the fine-tuning performance on downstream tasks, surpassing PiSSA in our experiments.

Some results are shown below.

1. The superiority of our decomposition method.

Plain SVD is the normal SVD on pre-trained weights, as adopted by PiSSA. This experiment discards the smallest $r$ components after different decomposition methods and tests the perplexity on Wiki and PTB. It indicates that our decomposition better compacts the associated task abilities into the principal components, and thus can better maintain the corresponding knowledge by freezing them in KPM, or better learn new abilities by adapting them in IPM.

2. Adaptation performance with knowledge-preserving mode (sample from NQopen, fine-tune on Math)

Method	Model	NQ open	GSM8k	Math	Avg.
Pre-trained	Llama-2-7b	14.99	-	-	-
LoRA	Llama-2-7b	1.27	42.68	5.88	16.61
CorDA (KPM)	Llama-2-7b	8.20	46.32	7.00	20.51
Pre-trained	Llama-2-13b	23.63	-	-	-
LoRA	Llama-2-13b	16.26	57.24	8.92	27.47
CorDA (KPM)	Llama-2-13b	19.86	59.29	9.62	29.59
Pre-trained	Llama-3-8b	13.41	-	-	-
LoRA	Llama-3-8b	8.75	72.33	24.04	35.04
CorDA (KPM)	Llama-3-8b	9.61	74.68	25.34	36.54
Pre-trained	Gemma-2-9b	12.85	-	-	-
LoRA	Gemma-2-9b	9.28	83.47	42.30	45.02
CorDA (KPM)	Gemma-2-9b	10.17	84.08	42.64	45.63

3. Adaptation performance with instruction-previewed mode (sample from Math, fine-tune on Math)

Method	Model	GSM8k	Math
LoRA	Llama-2-7b	42.68	5.88
PiSSA	Llama-2-7b	51.63	7.32
CorDA (IPM)	Llama-2-7b	53.45	8.64
LoRA	Llama-2-13b	57.24	8.92
PiSSA	Llama-2-13b	60.88	11.08
CorDA (IPM)	Llama-2-13b	62.47	11.54
LoRA	Gemma-2-9b	83.47	42.30
PiSSA	Gemma-2-9b	84.23	43.52
CorDA (IPM)	Gemma-2-9b	84.45	43.88

[BenjaminBossan]

Thanks for this PR to add CorDA to PEFT.

I don't have time to do a proper review this week, so I just did a very quick check and added some comments. I'll do a proper review next week.

[BenjaminBossan]

Let's raise a ValueError here and below.

[BenjaminBossan]

Let's not use asserts but instead raise proper errors with nice error messages.

[BenjaminBossan]

Let's add the license notice.

[5eqn]

@BenjaminBossan Thanks for your review! As @iboing 's collaborator, I've revised the code according to your comments.

[iboing]

@BenjaminBossan Thank you for your review!