After this PR: #10298

Objectives multi:softprob and multi:softmax (which correspond to multinomial logistic) calculate an intercept based on averaging values across all classes.

But this has no effect since adding a constant base score to all columns does not change the result after applying a softmax transformations - that is: softmax(a, b, c...) == softmax(a + const, b + const, c + const, ...).

It could skip the calculation altogether in such case.