Refine the Eigen usage for CPU implementation.

paddle/operators/momentum_op.h

@@ -44,15 +44,11 @@ class MomentumOpKernel : public framework::OpKernel<T> {
     auto g = framework::EigenVector<T>::Flatten(*grad);
     auto* lr = learning_rate->data<T>();
 
-    auto place = ctx.GetEigenDevice<platform::CPUPlace>();
-
-    Eigen::DSizes<int, 1> grad_dsize(grad->numel());
-
-    v_out.device(place) = v * mu + g;
+    v_out = v * mu + g;
     if (use_nesterov) {
-      p_out.device(place) = p - (g - v_out * mu) * lr[0];
+      p_out = p - (g - v_out * mu) * lr[0];
     } else {
-      p_out.device(place) = p - lr[0] * v_out;
+      p_out = p - lr[0] * v_out;
     }
   }
 };