Optimal threshold (tau) computation for 1.5-entmax

Usage

get_tau(input, dim = -1L, k = NULL)

input: The input tensor to compute thresholds over.
dim: The dimension along which to apply 1.5-entmax. Default is -1.
k: The number of largest elements to partial-sort over. For optimal performance, should be slightly bigger than the expected number of non-zeros in the solution. If the solution is more than k-sparse, this function is recursively called with a 2*k schedule. If NULL, full sorting is performed from the beginning. Default is NULL.

The threshold value for each vector, with all but the dim dimension intact.