convert .ml.tanh to a composition to match .ml.sigmoid

ml.q

@@ -850,7 +850,7 @@ linear:(::)                                   / linear
 dlinear:{1f+0f*$[99h=type x;x`z;x]}           / linear gradient
 sigmoid:1f%1f+exp neg::                       / sigmoid
 dsigmoid:{x*1f-x:$[99h=type x;x`a;sigmoid x]} / sigmoid gradient
-tanh:{1f-2f%1f+exp 2f*x}                      / hyperbolic tangent
+tanh:1f-2f%1f+exp 2f*                         / hyperbolic tangent
 dtanh:{1f-x*x:$[99h=type x;x`a;tanh x]}       / hyperbolic tangent gradient
 relu:0f|                              / rectified linear unit
 drelu:{"f"$0f<=$[99h=type x;x`z;x]}   / rectified linear unit gradient