Update cuda kernel and doc.

Author: qingqing01

paddle/operators/momentum_op.cc

@@ -71,8 +71,12 @@ class MomentumOpMaker : public framework::OpProtoAndCheckerMaker {
              "(Tensor, default Tensor<float>) "
              "Input learning rate");
 
-    AddOutput("ParamOut", "(Tensor) Output updated parameter");
-    AddOutput("VelocityOut", "(Tensor) Output updated velocity");
+    AddOutput("ParamOut",
+              "(Tensor) This output is updated parameter. "
+              "It shared memory with Input(Param).");
+    AddOutput("VelocityOut",
+              "(Tensor) This output is updated velocity. "
+              "It shared memory with Input(Velocity).");
 
     AddAttr<float>("mu", "(float) Momentum coefficient");
     AddAttr<bool>("use_nesterov",

paddle/operators/momentum_op.cu

@@ -29,7 +29,7 @@ __global__ void MomentumKernel(const T* p, const T* g, const T* v,
       T g_val = g[i];
       T v_new = v[i] * mu + g_val;
       v_out[i] = v_new;
-      p_out[i] = p[i] - g_val * lr + v_new * mu * lr;
+      p_out[i] = p[i] - (g_val - v_new * mu) * lr;
     }
   } else {
     for (int i = blockIdx.x * blockDim.x + threadIdx.x; i < num;