Returns the logarithm of the cumulative summation of the exponentiation of
elements of input
in the dimension dim
.
For summation index \(j\) given by dim
and other indices \(i\), the result is
$$
\mbox{logcumsumexp}(x)_{ij} = \log \sum\limits_{j=0}^{i} \exp(x_{ij})
$$