add sample code for summary (#3571) (#3573)

Author: LielinJiang

docs/api/paddle/summary_cn.rst

@@ -78,3 +78,17 @@ summary
     # ---------------------------------------------------------------------------
     # {'total_params': 61610, 'trainable_params': 61610}
 
+    # multi input demo
+    class LeNetMultiInput(LeNet):
+        def forward(self, inputs, y):
+            x = self.features(inputs)
+            if self.num_classes > 0:
+                x = paddle.flatten(x, 1)
+                x = self.fc(x + y)
+            return x
+    
+    lenet_multi_input = LeNetMultiInput()
+    params_info = paddle.summary(lenet_multi_input, [(1, 1, 28, 28), (1, 400)], 
+                                ['float32', 'float32'])
+    print(params_info)
+