docstr: Low-level feature pyramid network.
docstr_tok: ["Low-level","feature","pyramid","network","."]
code: def _low_level_fpn(self):
      """
      Low-level feature pyramid network.
      """
  
      def fpn(up_from, up_to):
          ch = up_to.shape[1]
          b_attr = ParamAttr(
              learning_rate=2.0, regularizer=L2Decay(0.0)
          )
          conv1 = fluid.layers.conv2d(
              up_from, ch, 1, act="relu", bias_attr=b_attr
          )
          if self.use_transposed_conv2d:
              w_attr = ParamAttr(
                  learning_rate=0.0,
                  regularizer=L2Decay(0.0),
                  initializer=Bilinear(),
              )
              upsampling = fluid.layers.conv2d_transpose(
                  conv1,
                  ch,
                  output_size=None,
                  filter_size=4,
                  padding=1,
                  stride=2,
                  groups=ch,
                  param_attr=w_attr,
                  bias_attr=False,
                  use_cudnn=True,
              )
          else:
              upsampling = fluid.layers.resize_bilinear(
                  conv1, out_shape=up_to.shape[2:]
              )
  
          conv2 = fluid.layers.conv2d(
              up_to, ch, 1, act="relu", bias_attr=b_attr
          )
          if self.is_infer:
              upsampling = fluid.layers.crop(upsampling, shape=conv2)
          # eltwise mul
          conv_fuse = upsampling * conv2
          return conv_fuse
  
      self.lfpn2_on_conv5 = fpn(self.conv6, self.conv5)
      self.lfpn1_on_conv4 = fpn(self.lfpn2_on_conv5, self.conv4)
      self.lfpn0_on_conv3 = fpn(self.lfpn1_on_conv4, self.conv3)
  
imports: from __future__ import absolute_import
  from __future__ import division
  from __future__ import print_function
  import numpy
  import six
  import paddle.fluid
  from paddle.fluid.param_attr import ParamAttr
  from paddle.fluid.initializer import Xavier
  from paddle.fluid.initializer import Constant
  from paddle.fluid.initializer import Bilinear
  from paddle.fluid.regularizer import L2Decay
label_desc: Use image node_id 5 for calling the PyramidBox obj's underlying member method code with example usage: obj._low_level_fpn() and returns: conv_fuse