fix the doc in elementwise_add elementwise_sub elementwise_mul elementwise_div (#1762)

Add the override function in the elementwise ops

doc/fluid/api_cn/layers_cn/elementwise_add_cn.rst

@@ -61,11 +61,12 @@ elementwise_add
     x = fluid.layers.data(name="x", shape=[3], dtype='float32')
     y = fluid.layers.data(name="y", shape=[3], dtype='float32')
     z = fluid.layers.elementwise_add(x, y)
+    # z = x + y
     place = fluid.CPUPlace()
     exe = fluid.Executor(place)
     z_value = exe.run(feed=gen_data(),
                         fetch_list=[z.name])
-    print(z_value) #[3., 8., 6.]
+    print(z_value) # [3., 8., 6.]
 
 **代码示例 2**
 
@@ -81,6 +82,7 @@ elementwise_add
     x = fluid.layers.data(name="x", shape=[2,3,4,5], dtype='float32')
     y = fluid.layers.data(name="y", shape=[3,4], dtype='float32')
     z = fluid.layers.elementwise_add(x, y, axis=1)
+    # z = x + y
     place = fluid.CPUPlace()
     exe = fluid.Executor(place)
     z_value = exe.run(feed=gen_data(),
@@ -100,6 +102,7 @@ elementwise_add
         }
     x = fluid.layers.data(name="x", shape=[2,3,4,5], dtype='float32')
     y = fluid.layers.data(name="y", shape=[3,4], dtype='float32')
+    # z = x + y
     z = fluid.layers.elementwise_add(x, y, axis=3)
     place = fluid.CPUPlace()
     exe = fluid.Executor(place)

doc/fluid/api_cn/layers_cn/elementwise_div_cn.rst

@@ -61,11 +61,12 @@ elementwise_div
     x = fluid.layers.data(name="x", shape=[3], dtype='float32')
     y = fluid.layers.data(name="y", shape=[3], dtype='float32')
     z = fluid.layers.elementwise_div(x, y)
+    # z = x / y
     place = fluid.CPUPlace()
     exe = fluid.Executor(place)
     z_value = exe.run(feed=gen_data(),
                         fetch_list=[z.name])
-    print(z_value) #[2., 0.6, 2.]
+    print(z_value) # [2., 0.6, 2.]
 
 **代码示例 2**
 
@@ -81,6 +82,7 @@ elementwise_div
     x = fluid.layers.data(name="x", shape=[2,3,4,5], dtype='float32')
     y = fluid.layers.data(name="y", shape=[3,4], dtype='float32')
     z = fluid.layers.elementwise_div(x, y, axis=1)
+    # z = x / y
     place = fluid.CPUPlace()
     exe = fluid.Executor(place)
     z_value = exe.run(feed=gen_data(),
@@ -101,6 +103,7 @@ elementwise_div
     x = fluid.layers.data(name="x", shape=[2,3,4,5], dtype='float32')
     y = fluid.layers.data(name="y", shape=[3,4], dtype='float32')
     z = fluid.layers.elementwise_div(x, y, axis=3)
+    # z = x / y
     place = fluid.CPUPlace()
     exe = fluid.Executor(place)
     z_value = exe.run(feed=gen_data(),

doc/fluid/api_cn/layers_cn/elementwise_mul_cn.rst

@@ -61,11 +61,12 @@ elementwise_mul
     x = fluid.layers.data(name="x", shape=[3], dtype='float32')
     y = fluid.layers.data(name="y", shape=[3], dtype='float32')
     z = fluid.layers.elementwise_mul(x, y)
+    # z = x * y
     place = fluid.CPUPlace()
     exe = fluid.Executor(place)
     z_value = exe.run(feed=gen_data(),
                         fetch_list=[z.name])
-    print(z_value) #[2., 15., 8.]
+    print(z_value) # [2., 15., 8.]
 
 **代码示例 2**
 
@@ -81,6 +82,7 @@ elementwise_mul
     x = fluid.layers.data(name="x", shape=[2,3,4,5], dtype='float32')
     y = fluid.layers.data(name="y", shape=[3,4], dtype='float32')
     z = fluid.layers.elementwise_mul(x, y, axis=1)
+    # z = x * y
     place = fluid.CPUPlace()
     exe = fluid.Executor(place)
     z_value = exe.run(feed=gen_data(),
@@ -101,6 +103,7 @@ elementwise_mul
     x = fluid.layers.data(name="x", shape=[2,3,4,5], dtype='float32')
     y = fluid.layers.data(name="y", shape=[3,4], dtype='float32')
     z = fluid.layers.elementwise_mul(x, y, axis=3)
+    # z = x * y
     place = fluid.CPUPlace()
     exe = fluid.Executor(place)
     z_value = exe.run(feed=gen_data(),

doc/fluid/api_cn/layers_cn/elementwise_sub_cn.rst

@@ -61,11 +61,12 @@ elementwise_sub
     x = fluid.layers.data(name="x", shape=[3], dtype='float32')
     y = fluid.layers.data(name="y", shape=[3], dtype='float32')
     z = fluid.layers.elementwise_sub(x, y)
+    # z = x - y
     place = fluid.CPUPlace()
     exe = fluid.Executor(place)
     z_value = exe.run(feed=gen_data(),
                         fetch_list=[z.name])
-    print(z_value) #[1., -2., 2.]
+    print(z_value) # [1., -2., 2.]
 
 **代码示例 2**
 
@@ -81,6 +82,7 @@ elementwise_sub
     x = fluid.layers.data(name="x", shape=[2,3,4,5], dtype='float32')
     y = fluid.layers.data(name="y", shape=[3,4], dtype='float32')
     z = fluid.layers.elementwise_sub(x, y, axis=1)
+    # z = x - y
     place = fluid.CPUPlace()
     exe = fluid.Executor(place)
     z_value = exe.run(feed=gen_data(),
@@ -101,6 +103,7 @@ elementwise_sub
     x = fluid.layers.data(name="x", shape=[2,3,4,5], dtype='float32')
     y = fluid.layers.data(name="y", shape=[3,4], dtype='float32')
     z = fluid.layers.elementwise_sub(x, y, axis=3)
+    # z = x - y
     place = fluid.CPUPlace()
     exe = fluid.Executor(place)
     z_value = exe.run(feed=gen_data(),