function?Y?=?vl_nnsoftmax（XdzdY）
%VL_NNSOFTMAX?CNN?softmax.
%???Y?=?VL_NNSOFTMAX（X）?applies?the?softmax?operator?the?data?X.?X
%???has?dimension?H?x?W?x?D?x?N?packing?N?arrays?of?W?x?H
%???D-dimensional?vectors.
%
%???D?can?be?thought?of?as?the?number?of?possible?classes?and?the
%???function?computes?the?softmax?along?the?D?dimension.?Often?W=H=1
%???but?this?is?not?a?requirement?as?the?operator?is?applied
%???convolutionally?at?all?spatial?locations.
%
%???DZDX?=?VL_NNSOFTMAX（X?DZDY）?c