  While many unsupervised learning models focus on one family of tasks, either
generative or discriminative, we explore the possibility of a unified
representation learner: a model which uses a single pre-training stage to
address both families of tasks simultaneously. We identify diffusion models as
a prime candidate. Diffusion models have risen to prominence as a
state-of-the-art method for image generation, denoising, inpainting,
super-resolution, manipulation, etc. Such models involve training a U-Net to
iteratively predict and remove noise, and the resulting model can synthesize
high fidelity, diverse, novel images. The U-Net architecture, as a
convolution-based architecture, generates a diverse set of feature
representations in the form of intermediate feature maps. We present our
findings that these embeddings are useful beyond the noise prediction task, as
they contain discriminative information and can also be leveraged for
classification. We explore optimal methods for extracting and using these
embeddings for classification tasks, demonstrating promising results on the
ImageNet classification task. We find that with careful feature selection and
pooling, diffusion models outperform comparable generative-discriminative
methods such as BigBiGAN for classification tasks. We investigate diffusion
models in the transfer learning regime, examining their performance on several
fine-grained visual classification datasets. We compare these embeddings to
those generated by competing architectures and pre-trainings for classification
tasks.
