  In zero-shot learning (ZSL) community, it is generally recognized that
transductive learning performs better than inductive one as the unseen-class
samples are also used in its training stage. How to generate pseudo labels for
unseen-class samples and how to use such usually noisy pseudo labels are two
critical issues in transductive learning. In this work, we introduce an
iterative co-training framework which contains two different base ZSL models
and an exchanging module. At each iteration, the two different ZSL models are
co-trained to separately predict pseudo labels for the unseen-class samples,
and the exchanging module exchanges the predicted pseudo labels, then the
exchanged pseudo-labeled samples are added into the training sets for the next
iteration. By such, our framework can gradually boost the ZSL performance by
fully exploiting the potential complementarity of the two models'
classification capabilities. In addition, our co-training framework is also
applied to the generalized ZSL (GZSL), in which a semantic-guided OOD detector
is proposed to pick out the most likely unseen-class samples before class-level
classification to alleviate the bias problem in GZSL. Extensive experiments on
three benchmarks show that our proposed methods could significantly outperform
about $31$ state-of-the-art ones.
