fix layer_norm accuracy (#29434)

paddle/fluid/operators/layer_norm_op.cu

@@ -135,7 +135,7 @@ __global__ void LayerNormForward(const T *x, const U *scale, const U *bias,
   }
   __syncthreads();
   mean_val = mean[blockIdx.x];
-  var_val = static_cast<U>(real_sqrt(var[blockIdx.x]) + epsilon);
+  var_val = static_cast<U>(real_sqrt(var[blockIdx.x] + epsilon));
 
   // Step 2: Calculate y
   if (scale != nullptr) {

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

@@ -211,7 +211,7 @@ class TestLayerNormOp(unittest.TestCase):
                                   for name in ['x', 'scale', 'bias', 'y@GRAD']
                               },
                               fetch_list=fetch_list)
-                self.__assert_close(y, out[0], "y", 1e-3)
+                self.__assert_close(y, out[0], "y")
                 self.__assert_close(mean, out[1], "mean")
                 self.__assert_close(variance, out[2], "variance", 1e-3)
                 self.__assert_close(x_grad, out[3], "x_grad")