Fix bug in reduce_op caused by PR #13534 (#13748)

* Fix bug in reduce_op caused by PR #13534
* Fix output shape and enhance unit test.
test=develop

paddle/fluid/operators/cub_reduce.h

@@ -22,6 +22,7 @@
 
 #include <cub/cub.cuh>  // NOLINT
 #include "paddle/fluid/framework/tensor.h"
+#include "paddle/fluid/framework/tensor_util.h"
 
 namespace paddle {
 namespace operators {
@@ -293,7 +294,12 @@ void TensorReduce(const framework::Tensor& x, framework::Tensor* y,
   }
   auto x_data = x.data<Tx>();
   auto y_data = y->mutable_data<Ty>(x.place());
-  if (reduce_num == 1) return;
+  if (reduce_num == 1) {
+    auto out_dims = y->dims();
+    framework::TensorCopy(x, y->place(), y);
+    y->Resize(out_dims);
+    return;
+  }
 
 #define CUB_BLOCK_DIM_CASE(block_dim)                                    \
   case block_dim: {                                                      \

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

@@ -243,5 +243,87 @@ class TestKeepDimReduceSumMultiAxises(OpTest):
         self.check_grad(['X'], 'Out')
 
 
+class TestReduceSumWithDimOne(OpTest):
+    def setUp(self):
+        self.op_type = "reduce_sum"
+        self.inputs = {'X': np.random.random((10, 1, 1)).astype("float64")}
+        self.attrs = {'dim': [1, 2], 'keep_dim': True}
+        self.outputs = {
+            'Out': self.inputs['X'].sum(axis=tuple(self.attrs['dim']),
+                                        keepdims=True)
+        }
+
+    def test_check_output(self):
+        self.check_output()
+
+    def test_check_grad(self):
+        self.check_grad(['X'], 'Out')
+
+
+class TestReduceSumWithNumelOne(OpTest):
+    def setUp(self):
+        self.op_type = "reduce_sum"
+        self.inputs = {'X': np.random.random((1, 1)).astype("float64")}
+        self.attrs = {'dim': [1], 'keep_dim': False}
+        self.outputs = {
+            'Out': self.inputs['X'].sum(axis=tuple(self.attrs['dim']),
+                                        keepdims=False)
+        }
+
+    def test_check_output(self):
+        self.check_output()
+
+    def test_check_grad(self):
+        self.check_grad(['X'], 'Out')
+
+
+class TestReduceMeanWithDimOne(OpTest):
+    def setUp(self):
+        self.op_type = "reduce_mean"
+        self.inputs = {'X': np.random.random((10, 1, 1)).astype("float64")}
+        self.attrs = {'dim': [1], 'keep_dim': False}
+        self.outputs = {
+            'Out': self.inputs['X'].mean(
+                axis=tuple(self.attrs['dim']), keepdims=False)
+        }
+
+    def test_check_output(self):
+        self.check_output()
+
+    def test_check_grad(self):
+        self.check_grad(['X'], 'Out')
+
+
+class TestReduceMeanWithNumelOne(OpTest):
+    def setUp(self):
+        self.op_type = "reduce_mean"
+        self.inputs = {'X': np.random.random((1, 1)).astype("float64")}
+        self.attrs = {'dim': [1], 'keep_dim': True}
+        self.outputs = {
+            'Out': self.inputs['X'].mean(
+                axis=tuple(self.attrs['dim']), keepdims=True)
+        }
+
+    def test_check_output(self):
+        self.check_output()
+
+    def test_check_grad(self):
+        self.check_grad(['X'], 'Out')
+
+
+class TestReduceAll(OpTest):
+    def setUp(self):
+        self.op_type = "reduce_sum"
+        self.inputs = {'X': np.random.random((1, 1, 1)).astype("float64")}
+        self.attrs = {'reduce_all': True, 'keep_dim': False}
+        self.outputs = {'Out': self.inputs['X'].sum()}
+
+    def test_check_output(self):
+        self.check_output()
+
+    def test_check_grad(self):
+        self.check_grad(['X'], 'Out')
+
+
 if __name__ == '__main__':
     unittest.main()