check extra_conv needed in mobilenet_v3 (#934)

ppdet/modeling/backbones/mobilenet_v3.py

@@ -393,6 +393,8 @@ class MobileNetV3(object):
             self.end_points.append(conv)
 
         # extra block
+        # check whether conv_extra is needed
+        if self.block_stride < max(self.feature_maps):
             conv_extra = self._conv_bn_layer(
                 conv,
                 filter_size=1,