class_docstr: Spatial Pyramid Pooling - Fast (SPPF) layer used in YOLOv5 by Glenn Jocher,
  equivalent to SPP(k=(5, 9, 13))
code: def forward(self, x):
      x = self.conv1(x)
      y1 = self.maxpooling(x)
      y2 = self.maxpooling(y1)
      y3 = self.maxpooling(y2)
      concats = paddle.concat([x, y1, y2, y3], axis=1)
      out = self.conv2(concats)
      return out
  
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 SPPFLayer obj's underlying member method code with example usage: obj.forward(x) and returns: out