fix vgg stop grad (#558)

* fix vgg stop grad

* beautify code

ppcls/modeling/architectures/vgg.py

@@ -68,10 +68,11 @@ class ConvBlock(nn.Layer):
 
 
 class VGGNet(nn.Layer):
-    def __init__(self, layers=11, class_dim=1000):
+    def __init__(self, layers=11, stop_grad_layers=0, class_dim=1000):
         super(VGGNet, self).__init__()
 
         self.layers = layers
+        self.stop_grad_layers = stop_grad_layers
         self.vgg_configure = {
             11: [1, 1, 2, 2, 2],
             13: [2, 2, 2, 2, 2],
@@ -89,6 +90,14 @@ class VGGNet(nn.Layer):
         self._conv_block_4 = ConvBlock(256, 512, self.groups[3], name="conv4_")
         self._conv_block_5 = ConvBlock(512, 512, self.groups[4], name="conv5_")
 
+        for idx, block in enumerate([
+                self._conv_block_1, self._conv_block_2, self._conv_block_3,
+                self._conv_block_4, self._conv_block_5
+        ]):
+            if self.stop_grad_layers >= idx + 1:
+                for param in block.parameters():
+                    param.trainable = False
+
         self._drop = Dropout(p=0.5, mode="downscale_in_infer")
         self._fc1 = Linear(
             7 * 7 * 512,