code: def forward(self, x):
      out = [x]
      for conv_layer in self.convs:
          out.append(conv_layer(out[-1]))
      return out
  
imports: import paddle
  import paddle.nn
  import paddle.nn.functional
  from ppdet.core.workspace import register
  from paddle.regularizer import L2Decay
  from paddle import ParamAttr
  from ..layers import AnchorGeneratorSSD
  from ..cls_utils import _get_class_default_kwargs
label_desc: Use image node_id 3 for calling the SSDExtraHead obj's underlying member method code with example usage: obj.forward(x) and returns: out