silu: fix cpu df

Author: swfsql

dfdx-core/src/tensor_ops/silu/cpu_kernel.rs

@@ -15,6 +15,6 @@ impl<F: num_traits::Float> UnaryDerivative<F> for super::SiLUKernelOp {
     #[inline(always)]
     fn df(&self, x: &F) -> F {
         let exp_nx = x.neg().exp();
-        F::one() + exp_nx + *x * exp_nx / (F::one() + exp_nx).powi(2)
+        (F::one() + exp_nx + *x * exp_nx) / (F::one() + exp_nx).powi(2)
     }
 }

dfdx-core/src/tensor_ops/silu/mod.rs

@@ -56,7 +56,7 @@ mod tests {
         let g = r.mean().backward();
         assert_close_to_literal!(
             g.get(&x),
-            [1.635814, 0.70433396, 0.4, 0.31289828, 0.26906452]
+            [-0.018156849, 0.014465898, 0.1, 0.1855341, 0.21815684]
         );
     }
 }