class_docstr: Spatial Pyramid Pooling (SPP) layer used in YOLOv3-SPP and YOLOX
code: def forward(self, x):
      x = self.conv1(x)
      x = paddle.concat(
          [x] + [mp(x) for mp in self.maxpoolings], axis=1
      )
      x = self.conv2(x)
      return x
  
imports: import paddle
  import paddle.nn
  import paddle.nn.functional
  from paddle import ParamAttr
  from paddle.regularizer import L2Decay
  from ppdet.core.workspace import register, serializable
  from ppdet.modeling.initializer import conv_init_
  from ..shape_spec import ShapeSpec
label_desc: Use image node_id 2 for calling the SPPLayer obj's underlying member method code with example usage: obj.forward(x) and returns: x