logsumexp is a convexity aware implementation of \(\log(\sum_i e^{x_i})\)

### Syntax

The convex logsumexp operator can be used to perform logistic regression, as illustrated in the example in the exponential cone tutorial

### Implementation

The operator is implemented as callback operator, except when Mosek 9, SCS or ECOS are used and the exponential cone description is exploited in a graph representation.