[yvdriess]

Surely you mean dense one-hot?

Technically, the sparse one-hot encoding is the most efficient in terms of memory footprint. You simply store the non-zero coordinates.

The problem in practice for GPUs is that sparse vector/matrix operations are too inefficient.

The whole point of something like this paper is to skip the entire 'densification' step and to directly deal with the sparse matrix input as a sparse matrix. The LSH is used in this paper improves on directly using SpMSpV, as that is also inefficient on CPUs, although to a lesser extent than GPUs.