fix the gradient bug for the topk v2

fix the gradient bug for the topk v2 

paddle/fluid/operators/top_k_function_cuda.h

@@ -335,6 +335,7 @@ __global__ void AssignGrad(T* x_grad, const int64_t* indices, const T* out_grad,
     for (size_t j = 0; j < cols; ++j) {
       x_grad[i * cols + j] = 0;
     }
+    __syncthreads();
     for (size_t j = 0; j < k; ++j) {
       size_t idx = indices[i * k + j];
       x_grad[i * cols + idx] = out_grad[i * k + j];
@@ -349,15 +350,16 @@ __global__ void AssignGradWithAxis(const T* grad_out, const int64_t* indices,
                                    int raw_height, int k) {
   // raw_height is the length of topk axis
   for (int i = blockIdx.x; i < pre; i += gridDim.x) {
-    const int& base_index = i * post * k;
-    const int& base_grad = i * post * raw_height;
+    int base_index = i * post * k;
+    int base_grad = i * post * raw_height;
     for (int j = threadIdx.x; j < raw_height * post; j += blockDim.x) {
       grad_in[base_grad + j] = static_cast<T>(0);
     }
+    __syncthreads();
     for (int j = threadIdx.x; j < k * post; j += blockDim.x) {
-      const int64_t idx_ij = indices[base_index + j];
-      const int64_t in_ij = base_grad + (idx_ij * post) + (j % post);
-      grad_in[in_ij] = grad_out[idx_ij];
+      int64_t idx_ij = indices[base_index + j];
+      int64_t in_ij = base_grad + (idx_ij * post) + (j % post);
+      grad_in[in_ij] = grad_out[base_index + j];
     }
   }
 }

python/paddle/fluid/tests/unittests/test_top_k_v2_op.py

@@ -64,34 +64,38 @@ class TestTopkOp(OpTest):
 
 
 class TestTopkOp1(TestTopkOp):
-    def init_args(self):
-        self.k = 3
-        self.axis = 0
-        self.largest = True
-
-
-class TestTopkOp2(TestTopkOp):
     def init_args(self):
         self.k = 3
         self.axis = 0
         self.largest = False
 
 
-class TestTopkOp3(TestTopkOp):
+class TestTopkOp2(TestTopkOp):
     def init_args(self):
         self.k = 4
         self.axis = 0
         self.largest = False
 
 
-class TestTopkOp4(TestTopkOp):
+class TestTopkOp3(OpTest):
     def init_args(self):
-        self.k = 4
-        self.axis = 0
-        self.largest = False
+        self.k = 6
+        self.axis = 1
+        self.largest = True
 
+    def setUp(self):
+        self.op_type = "top_k_v2"
+        self.dtype = np.float64
+        self.input_data = np.random.rand(16, 100)
+        self.init_args()
+        self.inputs = {'X': self.input_data}
+        self.attrs = {'k': self.k, 'axis': self.axis, 'largest': self.largest}
+        output, indices = numpy_topk(
+            self.input_data, axis=self.axis, k=self.k, largest=self.largest)
+        self.outputs = {'Out': output, 'Indices': indices}
 
-class TestTopkOp5(TestTopkOp):
+
+class TestTopkOp4(TestTopkOp):
     def init_args(self):
         self.k = 3
         self.axis = 1
@@ -109,7 +113,7 @@ class TestTopkOp5(TestTopkOp):
         self.outputs = {'Out': output, 'Indices': indices}
 
 
-class TestTopkOp6(TestTopkOp):
+class TestTopkOp5(TestTopkOp):
     def init_args(self):
         self.k = 3
         self.axis = 1