Add the affine transformation.

src/op.rs

@@ -2,6 +2,12 @@ use crate::Tensor;
 
 pub(crate) enum Op {
     Add(Tensor, Tensor),
+    #[allow(dead_code)] // add is currently unused.
+    Affine {
+        arg: Tensor,
+        mul: f64,
+        add: f64,
+    },
     Mul(Tensor, Tensor),
     Sqr(Tensor),
     Sqrt(Tensor),