  Conventionally, during the knowledge distillation process (e.g. feature
distillation), an additional projector is often required to perform feature
transformation due to the dimension mismatch between the teacher and the
student networks. Interestingly, we discovered that even if the student and the
teacher have the same feature dimensions, adding a projector still helps to
improve the distillation performance. In addition, projectors even improve
logit distillation if we add them to the architecture too. Inspired by these
surprising findings and the general lack of understanding of the projectors in
the knowledge distillation process from existing literature, this paper
investigates the implicit role that projectors play but so far have been
overlooked. Our empirical study shows that the student with a projector (1)
obtains a better trade-off between the training accuracy and the testing
accuracy compared to the student without a projector when it has the same
feature dimensions as the teacher, (2) better preserves its similarity to the
teacher beyond shallow and numeric resemblance, from the view of Centered
Kernel Alignment (CKA), and (3) avoids being over-confident as the teacher does
at the testing phase. Motivated by the positive effects of projectors, we
propose a projector ensemble-based feature distillation method to further
improve distillation performance. Despite the simplicity of the proposed
strategy, empirical results from the evaluation of classification tasks on
benchmark datasets demonstrate the superior classification performance of our
method on a broad range of teacher-student pairs and verify from the aspects of
CKA and model calibration that the student's features are of improved quality
with the projector ensemble design.
