change AdamW decay parameter to work like the torch AdamW decay parameter

It is now relative to Adam learning rate `alpha*sched`.
Before that it was relative to `sched` only.

`alpha` being the maximum learning rate and `sched` being a scaling parameter in [0..1]

ggml.c

@@ -17351,8 +17351,8 @@ static enum ggml_opt_result ggml_opt_adam(
 
     // constants
     const float sched = params.adam.sched;
-    const float decay = params.adam.decay * sched;
     const float alpha = params.adam.alpha * sched;
+    const float decay = params.adam.decay * alpha;
     const float beta1 = params.adam.beta1;
     const float beta2 = params.adam.beta2;
     const float eps   = params.adam.eps;