rewrite the sigmoid_focal_loss code example (#24889)

* rewrite the sigmoid_focal_loss code example. test=develop

* fix spelling mistake in comments of code example.test=develop

python/paddle/fluid/layers/detection.py

@@ -510,16 +510,73 @@ def sigmoid_focal_loss(x, label, fg_num, gamma=2.0, alpha=0.25):
     Examples:
         .. code-block:: python
 
+            import numpy as np
             import paddle.fluid as fluid
             
-            input = fluid.data(name='data', shape=[10,80], dtype='float32')
-            label = fluid.data(name='label', shape=[10,1], dtype='int32')
-            fg_num = fluid.data(name='fg_num', shape=[1], dtype='int32')
-            loss = fluid.layers.sigmoid_focal_loss(x=input,
-                                                   label=label,
-                                                   fg_num=fg_num,
-                                                   gamma=2.0,
-                                                   alpha=0.25)
+            num_classes = 10  # exclude background
+            image_width = 16
+            image_height = 16
+            batch_size = 32
+            max_iter = 20
+            
+            
+            def gen_train_data():
+                x_data = np.random.uniform(0, 255, (batch_size, 3, image_height,
+                                                    image_width)).astype('float64')
+                label_data = np.random.randint(0, num_classes,
+                                               (batch_size, 1)).astype('int32')
+                return {"x": x_data, "label": label_data}
+            
+            
+            def get_focal_loss(pred, label, fg_num, num_classes):
+                pred = fluid.layers.reshape(pred, [-1, num_classes])
+                label = fluid.layers.reshape(label, [-1, 1])
+                label.stop_gradient = True
+                loss = fluid.layers.sigmoid_focal_loss(
+                    pred, label, fg_num, gamma=2.0, alpha=0.25)
+                loss = fluid.layers.reduce_sum(loss)
+                return loss
+            
+            
+            def build_model(mode='train'):
+                x = fluid.data(name="x", shape=[-1, 3, -1, -1], dtype='float64')
+                output = fluid.layers.pool2d(input=x, pool_type='avg', global_pooling=True)
+                output = fluid.layers.fc(
+                    input=output,
+                    size=num_classes,
+                    # Notice: size is set to be the number of target classes (excluding backgorund)
+                    # because sigmoid activation will be done in the sigmoid_focal_loss op.
+                    act=None)
+                if mode == 'train':
+                    label = fluid.data(name="label", shape=[-1, 1], dtype='int32')
+                    # Obtain the fg_num needed by the sigmoid_focal_loss op:
+                    # 0 in label represents background, >=1 in label represents foreground,
+                    # find the elements in label which are greater or equal than 1, then
+                    # computed the numbers of these elements.
+                    data = fluid.layers.fill_constant(shape=[1], value=1, dtype='int32')
+                    fg_label = fluid.layers.greater_equal(label, data)
+                    fg_label = fluid.layers.cast(fg_label, dtype='int32')
+                    fg_num = fluid.layers.reduce_sum(fg_label)
+                    fg_num.stop_gradient = True
+                    avg_loss = get_focal_loss(output, label, fg_num, num_classes)
+                    return avg_loss
+                else:
+                    # During evaluating or testing phase,
+                    # output of the final fc layer should be connected to a sigmoid layer.
+                    pred = fluid.layers.sigmoid(output)
+                    return pred
+            
+            
+            loss = build_model('train')
+            moment_optimizer = fluid.optimizer.MomentumOptimizer(
+                learning_rate=0.001, momentum=0.9)
+            moment_optimizer.minimize(loss)
+            place = fluid.CPUPlace()
+            exe = fluid.Executor(place)
+            exe.run(fluid.default_startup_program())
+            for i in range(max_iter):
+                outs = exe.run(feed=gen_train_data(), fetch_list=[loss.name])
+                print(outs)
     """
 
     check_variable_and_dtype(x, 'x', ['float32', 'float64'],