remove fluid in default_main_program doc, test=document_fix (#28941)

python/paddle/fluid/framework.py

@@ -5400,7 +5400,7 @@ def default_main_program():
     This API can be used to get ``default main program`` which store the 
     descriptions of Ops and tensors.
     
-    For example ``z = paddle.fluid.layers.elementwise_add(x, y)`` will create a new ``elementwise_add`` 
+    For example ``z = paddle.add(x, y)`` will create a new ``add`` 
     Op and a new ``z`` tensor, and they will be recorded in ``default main program`` . 
 
     The ``default main program`` is the default value for ``Program`` parameter in 
@@ -5419,33 +5419,14 @@ def default_main_program():
 
             paddle.enable_static()
             # Sample Network:
-            data = paddle.static.data(name='image', shape=[None, 3, 224, 224], dtype='float32')
-            label = paddle.static.data(name='label', shape=[None, 1], dtype='int64')
-            
-            conv1 = paddle.static.nn.conv2d(data, 4, 5, 1, act=None)
-            bn1 = paddle.static.nn.batch_norm(conv1, act='relu')
-            pool1 = paddle.fluid.layers.pool2d(bn1, 2, 'max', 2)
-            conv2 = paddle.static.nn.conv2d(pool1, 16, 5, 1, act=None)
-            bn2 = paddle.static.nn.batch_norm(conv2, act='relu')
-            pool2 = paddle.fluid.layers.pool2d(bn2, 2, 'max', 2)
-            
-            fc1 = paddle.static.nn.fc(x=pool2, size=50, activation='relu')
-            fc2 = paddle.static.nn.fc(x=fc1, size=102, activation='softmax')
-            
-            loss = paddle.nn.functional.loss.cross_entropy(input=fc2, label=label)
-            loss = paddle.mean(loss)
-            opt = paddle.optimizer.Momentum(
-                learning_rate=0.1,
-                momentum=0.9,
-                weight_decay=paddle.regularizer.L2Decay(1e-4))
-            opt.minimize(loss)
+            x = paddle.static.data(name='x', shape=[100, 100], dtype='float32')
+            y = paddle.static.data(name='x', shape=[100, 100], dtype='float32')
+            out = paddle.add(x, y)
 
             #print the number of blocks in the program, 1 in this case
-            print(paddle.static.default_main_program().num_blocks) #[1]
-
-            #print the description of variable 'image'
+            print(paddle.static.default_main_program().num_blocks) # 1
+            #print the default_main_program
             print(paddle.static.default_main_program())
-
     """
     return _main_program_