  In continual learning (CL), a learner is faced with a sequence of tasks,
arriving one after the other, and the goal is to remember all the tasks once
the continual learning experience is finished. The prior art in CL uses
episodic memory, parameter regularization or extensible network structures to
reduce interference among tasks, but in the end, all the approaches learn
different tasks in a joint vector space. We believe this invariably leads to
interference among different tasks. We propose to learn tasks in different
(low-rank) vector subspaces that are kept orthogonal to each other in order to
minimize interference. Further, to keep the gradients of different tasks coming
from these subspaces orthogonal to each other, we learn isometric mappings by
posing network training as an optimization problem over the Stiefel manifold.
To the best of our understanding, we report, for the first time, strong results
over experience-replay baseline with and without memory on standard
classification benchmarks in continual learning. The code is made publicly
available.
