FrameQuant: Flexible Low-Bit Quantization for Transformers
Harshavardhan Adepu Zhanpeng Zeng Li Zhang Vikas Singh
Transformers are the backbone of powerful foundation models for many Vision
and Natural Language Processing tasks. But their compute and memory/storage
footprint is large, and so, serving such models is expensive often requiring
high-end hardware. To mitigate this difficulty, Post-Training Quantization
seeks to modify a pre-trained model and quantize it to eight bits or lower,
significantly boosting compute/memory/latency efficiency. Such models have been
successfully quantized to four bits with some performance loss. In this work,
we outline a simple scheme to quantize Transformer-based models to just two
bits (plus some overhead) with only a small drop in accuracy. Key to our
formulation is a concept borrowed from Harmonic analysis called Fusion Frames.
Our main finding is that the quantization must take place not in the original
weight space, but instead in the Fusion Frame representations. If quantization
is interpreted as the addition of noise, our casting of the problem allows
invoking an extensive body of known consistent recovery and noise robustness
guarantees. Further, if desired, de-noising filters are known in closed form.
We show empirically, via a variety of experiments, that (almost) two-bit
quantization for Transformer models promises sizable efficiency gains.