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一起来看看Efficientdet的keras实现吧，顺便训练一下自己的数据。
最近，谷歌大脑 Mingxing Tan、Ruoming Pang 和 Quoc V. Le 提出新架构 EfficientDet，结合 EfficientNet（同样来自该团队）和新提出的 BiFPN，实现新的 SOTA 结果。
https://github.com/bubbliiiing/efficientdet-keras
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1、主干网络介绍
Efficientdet采用Efficientnet作为主干特征提取网络。EfficientNet-B0对应Efficientdet-D0；EfficientNet-B1对应Efficientdet-D1；以此类推。
EfficientNet模型具有很独特的特点，这个特点是参考其它优秀神经网络设计出来的。经典的神经网络特点如下：
1、利用残差神经网络增大神经网络的深度，通过更深的神经网络实现特征提取。
2、改变每一层提取的特征层数，实现更多层的特征提取，得到更多的特征，提升宽度。
3、通过增大输入图片的分辨率也可以使得网络可以学习与表达的东西更加丰富，有利于提高精确度。
EfficientNet就是将这三个特点结合起来，通过一起缩放baseline模型（MobileNet中就通过缩放α实现缩放模型，不同的α有不同的模型精度，α=1时为baseline模型；ResNet其实也是有一个baseline模型，在baseline的基础上通过改变图片的深度实现不同的模型实现），同时调整深度、宽度、输入图片的分辨率完成一个优秀的网络设计。
在EfficientNet模型中，其使用一组固定的缩放系数统一缩放网络深度、宽度和分辨率。
假设想使用 2N倍的计算资源，我们可以简单的对网络深度扩大αN倍、宽度扩大βN 、图像尺寸扩大γN倍，这里的α,β,γ都是由原来的小模型上做微小的网格搜索决定的常量系数。
如图为EfficientNet的设计思路，从三个方面同时拓充网络的特性。
本博客以Efficientnet-B0和Efficientdet-D0为例，进行Efficientdet的解析。
Efficientnet-B0由1个Stem+16个大Blocks堆叠构成，16个大Blocks可以分为1、2、2、3、3、4、1个Block。Block的通用结构如下，其总体的设计思路是Inverted residuals结构和残差结构，在3x3或者5x5网络结构前利用1x1卷积升维，在3x3或者5x5网络结构后增加了一个关于通道的注意力机制，最后利用1x1卷积降维后增加一个大残差边。
整体结构如下：
最终获得三个有效特征层传入到BIFPN当中进行下一步的操作。
from __future__ import absolute_import
from __future__ import division
from __future__ import print_function
import os
import json
import math
import string
import collections
import numpy as np
from keras import backend
from six.moves import xrange
from nets.layers import BatchNormalization
from keras import layers
BASE_WEIGHTS_PATH = (
'https://github.com/Callidior/keras-applications/'
'releases/download/efficientnet/')
WEIGHTS_HASHES = {
'efficientnet-b0': ('163292582f1c6eaca8e7dc7b51b01c61'
'5b0dbc0039699b4dcd0b975cc21533dc',
'c1421ad80a9fc67c2cc4000f666aa507'
'89ce39eedb4e06d531b0c593890ccff3'),
'efficientnet-b1': ('d0a71ddf51ef7a0ca425bab32b7fa7f1'
'6043ee598ecee73fc674d9560c8f09b0',
'75de265d03ac52fa74f2f510455ba64f'
'9c7c5fd96dc923cd4bfefa3d680c4b68'),
'efficientnet-b2': ('bb5451507a6418a574534aa76a91b106'
'f6b605f3b5dde0b21055694319853086',
'433b60584fafba1ea3de07443b74cfd3'
'2ce004a012020b07ef69e22ba8669333'),
'efficientnet-b3': ('03f1fba367f070bd2545f081cfa7f3e7'
'6f5e1aa3b6f4db700f00552901e75ab9',
'c5d42eb6cfae8567b418ad3845cfd63a'
'a48b87f1bd5df8658a49375a9f3135c7'),
'efficientnet-b4': ('98852de93f74d9833c8640474b2c698d'
'b45ec60690c75b3bacb1845e907bf94f',
'7942c1407ff1feb34113995864970cd4'
'd9d91ea64877e8d9c38b6c1e0767c411'),
'efficientnet-b5': ('30172f1d45f9b8a41352d4219bf930ee'
'3339025fd26ab314a817ba8918fefc7d',
'9d197bc2bfe29165c10a2af8c2ebc675'
'07f5d70456f09e584c71b822941b1952'),
'efficientnet-b6': ('f5270466747753485a082092ac9939ca'
'a546eb3f09edca6d6fff842cad938720',
'1d0923bb038f2f8060faaf0a0449db4b'
'96549a881747b7c7678724ac79f427ed'),
'efficientnet-b7': ('876a41319980638fa597acbbf956a82d'
'10819531ff2dcb1a52277f10c7aefa1a',
'60b56ff3a8daccc8d96edfd40b204c11'
'3e51748da657afd58034d54d3cec2bac')
}
BlockArgs = collections.namedtuple('BlockArgs', [
'kernel_size', 'num_repeat', 'input_filters', 'output_filters',
'expand_ratio', 'id_skip', 'strides', 'se_ratio'
])
# defaults will be a public argument for namedtuple in Python 3.7
# https://docs.python.org/3/library/collections.html#collections.namedtuple
BlockArgs.__new__.__defaults__ = (None,) * len(BlockArgs._fields)
DEFAULT_BLOCKS_ARGS = [
BlockArgs(kernel_size=3, num_repeat=1, input_filters=32, output_filters=16,
expand_ratio=1, id_skip=True, strides=[1, 1], se_ratio=0.25),
BlockArgs(kernel_size=3, num_repeat=2, input_filters=16, output_filters=24,
expand_ratio=6, id_skip=True, strides=[2, 2], se_ratio=0.25),
BlockArgs(kernel_size=5, num_repeat=2, input_filters=24, output_filters=40,
expand_ratio=6, id_skip=True, strides=[2, 2], se_ratio=0.25),
BlockArgs(kernel_size=3, num_repeat=3, input_filters=40, output_filters=80,
expand_ratio=6, id_skip=True, strides=[2, 2], se_ratio=0.25),
BlockArgs(kernel_size=5, num_repeat=3, input_filters=80, output_filters=112,
expand_ratio=6, id_skip=True, strides=[1, 1], se_ratio=0.25),
BlockArgs(kernel_size=5, num_repeat=4, input_filters=112, output_filters=192,
expand_ratio=6, id_skip=True, strides=[2, 2], se_ratio=0.25),
BlockArgs(kernel_size=3, num_repeat=1, input_filters=192, output_filters=320,
expand_ratio=6, id_skip=True, strides=[1, 1], se_ratio=0.25)
]
CONV_KERNEL_INITIALIZER = {
'class_name': 'VarianceScaling',
'config': {
'scale': 2.0,
'mode': 'fan_out',
# EfficientNet actually uses an untruncated normal distribution for
# initializing conv layers, but keras.initializers.VarianceScaling use
# a truncated distribution.
# We decided against a custom initializer for better serializability.
'distribution': 'normal'
}
}
DENSE_KERNEL_INITIALIZER = {
'class_name': 'VarianceScaling',
'config': {
'scale': 1. / 3.,
'mode': 'fan_out',
'distribution': 'uniform'
}
}
def get_swish():
def swish(x):
return x * backend.sigmoid(x)
return swish
def get_dropout():
class FixedDropout(layers.Dropout):
def _get_noise_shape(self, inputs):
if self.noise_shape is None:
return self.noise_shape
symbolic_shape = backend.shape(inputs)
noise_shape = [symbolic_shape[axis] if shape is None else shape
for axis, shape in enumerate(self.noise_shape)]
return tuple(noise_shape)
return FixedDropout
def round_filters(filters, width_coefficient, depth_divisor):
filters *= width_coefficient
new_filters = int(filters + depth_divisor / 2) // depth_divisor * depth_divisor
new_filters = max(depth_divisor, new_filters)
if new_filters < 0.9 * filters:
new_filters += depth_divisor
return int(new_filters)
def round_repeats(repeats, depth_coefficient):
return int(math.ceil(depth_coefficient * repeats))
def mb_conv_block(inputs, block_args, activation, drop_rate=None, prefix='', freeze_bn=False):
has_se = (block_args.se_ratio is not None) and (0 < block_args.se_ratio <= 1)
bn_axis = 3
Dropout = get_dropout()
filters = block_args.input_filters * block_args.expand_ratio
if block_args.expand_ratio != 1:
x = layers.Conv2D(filters, 1,
padding='same',
use_bias=False,
kernel_initializer=CONV_KERNEL_INITIALIZER,
name=prefix + 'expand_conv')(inputs)
x = layers.BatchNormalization(axis=bn_axis, name=prefix + 'expand_bn')(x)
x = layers.Activation(activation, name=prefix + 'expand_activation')(x)
else:
x = inputs
# Depthwise Convolution
x = layers.DepthwiseConv2D(block_args.kernel_size,
strides=block_args.strides,
padding='same',
use_bias=False,
depthwise_initializer=CONV_KERNEL_INITIALIZER,
name=prefix + 'dwconv')(x)
x = layers.BatchNormalization(axis=bn_axis, name=prefix + 'bn')(x)
x = layers.Activation(activation, name=prefix + 'activation')(x)
# Squeeze and Excitation phase
if has_se:
num_reduced_filters = max(1, int(
block_args.input_filters * block_args.se_ratio
))
se_tensor = layers.GlobalAveragePooling2D(name=prefix + 'se_squeeze')(x)
target_shape = (1, 1, filters) if backend.image_data_format() == 'channels_last' else (filters, 1, 1)
se_tensor = layers.Reshape(target_shape, name=prefix + 'se_reshape')(se_tensor)
se_tensor = layers.Conv2D(num_reduced_filters, 1,
activation=activation,
padding='same',
use_bias=True,
kernel_initializer=CONV_KERNEL_INITIALIZER,
name=prefix + 'se_reduce')(se_tensor)
se_tensor = layers.Conv2D(filters, 1,
activation='sigmoid',
padding='same',
use_bias=True,
kernel_initializer=CONV_KERNEL_INITIALIZER,
name=prefix + 'se_expand')(se_tensor)
if backend.backend() == 'theano':
# For the Theano backend, we have to explicitly make
# the excitation weights broadcastable.
pattern = ([True, True, True, False] if backend.image_data_format() == 'channels_last'
else [True, False, True, True])
se_tensor = layers.Lambda(
lambda x: backend.pattern_broadcast(x, pattern),
name=prefix + 'se_broadcast')(se_tensor)
x = layers.multiply([x, se_tensor], name=prefix + 'se_excite')
# Output phase
x = layers.Conv2D(block_args.output_filters, 1,
padding='same',
use_bias=False,
kernel_initializer=CONV_KERNEL_INITIALIZER,
name=prefix + 'project_conv')(x)
# x = BatchNormalization(freeze=freeze_bn, axis=bn_axis, name=prefix + 'project_bn')(x)
x = layers.BatchNormalization(axis=bn_axis, name=prefix + 'project_bn')(x)
if block_args.id_skip and all(
s == 1 for s in block_args.strides
) and block_args.input_filters == block_args.output_filters:
if drop_rate and (drop_rate > 0):
x = Dropout(drop_rate,
noise_shape=(None, 1, 1, 1),
name=prefix + 'drop')(x)
x = layers.add([x, inputs], name=prefix + 'add')
return x
def EfficientNet(width_coefficient,
depth_coefficient,
default_resolution,
dropout_rate=0.2,
drop_connect_rate=0.2,
depth_divisor=8,
blocks_args=DEFAULT_BLOCKS_ARGS,
model_name='efficientnet',
include_top=True,
weights='imagenet',
input_tensor=None,
input_shape=None,
pooling=None,
classes=1000,
freeze_bn=False,
**kwargs):
features = []
if input_tensor is None:
img_input = layers.Input(shape=input_shape)
else:
img_input = input_tensor
bn_axis = 3
activation = get_swish(**kwargs)
# Build stem
x = img_input
x = layers.Conv2D(round_filters(32, width_coefficient, depth_divisor), 3,
strides=(2, 2),
padding='same',
use_bias=False,
kernel_initializer=CONV_KERNEL_INITIALIZER,
name='stem_conv')(x)
# x = BatchNormalization(freeze=freeze_bn, axis=bn_axis, name='stem_bn')(x)
x = layers.BatchNormalization(axis=bn_axis, name='stem_bn')(x)
x = layers.Activation(activation, name='stem_activation')(x)
# Build blocks
num_blocks_total = sum(block_args.num_repeat for block_args in blocks_args)
block_num = 0
for idx, block_args in enumerate(blocks_args):
assert block_args.num_repeat > 0
# Update block input and output filters based on depth multiplier.
block_args = block_args._replace(
input_filters=round_filters(block_args.input_filters,
width_coefficient, depth_divisor),
output_filters=round_filters(block_args.output_filters,
width_coefficient, depth_divisor),
num_repeat=round_repeats(block_args.num_repeat, depth_coefficient))
# The first block needs to take care of stride and filter size increase.
drop_rate = drop_connect_rate * float(block_num) / num_blocks_total
x = mb_conv_block(x, block_args,
activation=activation,
drop_rate=drop_rate,
prefix='block{}a_'.format(idx + 1),
freeze_bn=freeze_bn
)
block_num += 1
if block_args.num_repeat > 1:
# pylint: disable=protected-access
block_args = block_args._replace(
input_filters=block_args.output_filters, strides=[1, 1])
# pylint: enable=protected-access
for bidx in xrange(block_args.num_repeat - 1):
drop_rate = drop_connect_rate * float(block_num) / num_blocks_total
block_prefix = 'block{}{}_'.format(
idx + 1,
string.ascii_lowercase[bidx + 1]
)
x = mb_conv_block(x, block_args,
activation=activation,
drop_rate=drop_rate,
prefix=block_prefix,
freeze_bn=freeze_bn
)
block_num += 1
if idx < len(blocks_args) - 1 and blocks_args[idx + 1].strides[0] == 2:
features.append(x)
elif idx == len(blocks_args) - 1:
features.append(x)
return features
def EfficientNetB0(include_top=True,
weights='imagenet',
input_tensor=None,
input_shape=None,
pooling=None,
classes=1000,
**kwargs):
return EfficientNet(1.0, 1.0, 224, 0.2,
model_name='efficientnet-b0',
include_top=include_top, weights=weights,
input_tensor=input_tensor, input_shape=input_shape,
pooling=pooling, classes=classes,
**kwargs)
def EfficientNetB1(include_top=True,
weights='imagenet',
input_tensor=None,
input_shape=None,
pooling=None,
classes=1000,
**kwargs):
return EfficientNet(1.0, 1.1, 240, 0.2,
model_name='efficientnet-b1',
include_top=include_top, weights=weights,
input_tensor=input_tensor, input_shape=input_shape,
pooling=pooling, classes=classes,
**kwargs)
def EfficientNetB2(include_top=True,
weights='imagenet',
input_tensor=None,
input_shape=None,
pooling=None,
classes=1000,
**kwargs):
return EfficientNet(1.1, 1.2, 260, 0.3,
model_name='efficientnet-b2',
include_top=include_top, weights=weights,
input_tensor=input_tensor, input_shape=input_shape,
pooling=pooling, classes=classes,
**kwargs)
def EfficientNetB3(include_top=True,
weights='imagenet',
input_tensor=None,
input_shape=None,
pooling=None,
classes=1000,
**kwargs):
return EfficientNet(1.2, 1.4, 300, 0.3,
model_name='efficientnet-b3',
include_top=include_top, weights=weights,
input_tensor=input_tensor, input_shape=input_shape,
pooling=pooling, classes=classes,
**kwargs)
def EfficientNetB4(include_top=True,
weights='imagenet',
input_tensor=None,
input_shape=None,
pooling=None,
classes=1000,
**kwargs):
return EfficientNet(1.4, 1.8, 380, 0.4,
model_name='efficientnet-b4',
include_top=include_top, weights=weights,
input_tensor=input_tensor, input_shape=input_shape,
pooling=pooling, classes=classes,
**kwargs)
def EfficientNetB5(include_top=True,
weights='imagenet',
input_tensor=None,
input_shape=None,
pooling=None,
classes=1000,
**kwargs):
return EfficientNet(1.6, 2.2, 456, 0.4,
model_name='efficientnet-b5',
include_top=include_top, weights=weights,
input_tensor=input_tensor, input_shape=input_shape,
pooling=pooling, classes=classes,
**kwargs)
def EfficientNetB6(include_top=True,
weights='imagenet',
input_tensor=None,
input_shape=None,
pooling=None,
classes=1000,
**kwargs):
return EfficientNet(1.8, 2.6, 528, 0.5,
model_name='efficientnet-b6',
include_top=include_top, weights=weights,
input_tensor=input_tensor, input_shape=input_shape,
pooling=pooling, classes=classes,
**kwargs)
def EfficientNetB7(include_top=True,
weights='imagenet',
input_tensor=None,
input_shape=None,
pooling=None,
classes=1000,
**kwargs):
return EfficientNet(2.0, 3.1, 600, 0.5,
model_name='efficientnet-b7',
include_top=include_top, weights=weights,
input_tensor=input_tensor, input_shape=input_shape,
pooling=pooling, classes=classes,
**kwargs)
2、BiFPN加强特征提取
BiFPN简单来讲是一个加强版本的FPN，上图是BiFPN，下图是普通的FPN，大家可以看到，与普通的FPN相比，BiFPN的FPN构建更加复杂，中间还增加了许多连接。
构建BiFPN可以分为多步：
1、获得P3_in、P4_in、P5_in、P6_in、P7_in，通过主干特征提取网络，我们已经可以获得P3、P4、P5，还需要进行两次下采样获得P6、P7。
P3、P4、P5在经过1x1卷积调整通道数后，就可以作为P3_in、P4_in、P5_in了，在构建BiFPN的第一步，需要构建两个P4_in、P5_in（原版是这样设计的）。
实现代码如下：
_, _, C3, C4, C5 = features
# 第一次BIFPN需要 下采样 与 降通道 获得 p3_in p4_in p5_in p6_in p7_in
#-----------------------------下采样 与 降通道----------------------------#
P3_in = C3
P3_in = layers.Conv2D(num_channels, kernel_size=1, padding='same',
name=f'fpn_cells/cell_{id}/fnode3/resample_0_0_8/conv2d')(P3_in)
P3_in = layers.BatchNormalization(momentum=MOMENTUM, epsilon=EPSILON,
name=f'fpn_cells/cell_{id}/fnode3/resample_0_0_8/bn')(P3_in)
P4_in = C4
P4_in_1 = layers.Conv2D(num_channels, kernel_size=1, padding='same',
name=f'fpn_cells/cell_{id}/fnode2/resample_0_1_7/conv2d')(P4_in)
P4_in_1 = layers.BatchNormalization(momentum=MOMENTUM, epsilon=EPSILON,
name=f'fpn_cells/cell_{id}/fnode2/resample_0_1_7/bn')(P4_in_1)
P4_in_2 = layers.Conv2D(num_channels, kernel_size=1, padding='same',
name=f'fpn_cells/cell_{id}/fnode4/resample_0_1_9/conv2d')(P4_in)
P4_in_2 = layers.BatchNormalization(momentum=MOMENTUM, epsilon=EPSILON,
name=f'fpn_cells/cell_{id}/fnode4/resample_0_1_9/bn')(P4_in_2)
P5_in = C5
P5_in_1 = layers.Conv2D(num_channels, kernel_size=1, padding='same',
name=f'fpn_cells/cell_{id}/fnode1/resample_0_2_6/conv2d')(P5_in)
P5_in_1 = layers.BatchNormalization(momentum=MOMENTUM, epsilon=EPSILON,
name=f'fpn_cells/cell_{id}/fnode1/resample_0_2_6/bn')(P5_in_1)
P5_in_2 = layers.Conv2D(num_channels, kernel_size=1, padding='same',
name=f'fpn_cells/cell_{id}/fnode5/resample_0_2_10/conv2d')(P5_in)
P5_in_2 = layers.BatchNormalization(momentum=MOMENTUM, epsilon=EPSILON,
name=f'fpn_cells/cell_{id}/fnode5/resample_0_2_10/bn')(P5_in_2)
P6_in = layers.Conv2D(num_channels, kernel_size=1, padding='same', name='resample_p6/conv2d')(C5)
P6_in = layers.BatchNormalization(momentum=MOMENTUM, epsilon=EPSILON, name='resample_p6/bn')(P6_in)
P6_in = layers.MaxPooling2D(pool_size=3, strides=2, padding='same', name='resample_p6/maxpool')(P6_in)
P7_in = layers.MaxPooling2D(pool_size=3, strides=2, padding='same', name='resample_p7/maxpool')(P6_in)
#-------------------------------------------------------------------------#
2、在获得P3_in、P4_in_1、P4_in_2、P5_in_1、P5_in_2、P6_in、P7_in之后需要对P7_in进行上采样，上采样后与P6_in堆叠获得P6_td；之后对P6_td进行上采样，上采样后与P5_in_1进行堆叠获得P5_td；之后对P5_td进行上采样，上采样后与P4_in_1进行堆叠获得P4_td；之后对P4_td进行上采样，上采样后与P3_in进行堆叠获得P3_out。
实现代码如下：
#--------------------------构建BIFPN的上下采样循环-------------------------#
P7_U = layers.UpSampling2D()(P7_in)
P6_td = wBiFPNAdd(name=f'fpn_cells/cell_{id}/fnode0/add')([P6_in, P7_U])
P6_td = layers.Activation(lambda x: tf.nn.swish(x))(P6_td)
P6_td = SeparableConvBlock(num_channels=num_channels, kernel_size=3, strides=1,
name=f'fpn_cells/cell_{id}/fnode0/op_after_combine5')(P6_td)
P6_U = layers.UpSampling2D()(P6_td)
P5_td = wBiFPNAdd(name=f'fpn_cells/cell_{id}/fnode1/add')([P5_in_1, P6_U])
P5_td = layers.Activation(lambda x: tf.nn.swish(x))(P5_td)
P5_td = SeparableConvBlock(num_channels=num_channels, kernel_size=3, strides=1,
name=f'fpn_cells/cell_{id}/fnode1/op_after_combine6')(P5_td)
P5_U = layers.UpSampling2D()(P5_td)
P4_td = wBiFPNAdd(name=f'fpn_cells/cell_{id}/fnode2/add')([P4_in_1, P5_U])
P4_td = layers.Activation(lambda x: tf.nn.swish(x))(P4_td)
P4_td = SeparableConvBlock(num_channels=num_channels, kernel_size=3, strides=1,
name=f'fpn_cells/cell_{id}/fnode2/op_after_combine7')(P4_td)
P4_U = layers.UpSampling2D()(P4_td)
P3_out = wBiFPNAdd(name=f'fpn_cells/cell_{id}/fnode3/add')([P3_in, P4_U])
P3_out = layers.Activation(lambda x: tf.nn.swish(x))(P3_out)
P3_out = SeparableConvBlock(num_channels=num_channels, kernel_size=3, strides=1,
name=f'fpn_cells/cell_{id}/fnode3/op_after_combine8')(P3_out)
#-------------------------------------------------------------------------#
3、在获得P3_out、P4_td、P4_in_2、P5_td、P5_in_2、P6_in、P6_td、P7_in之后，之后需要对P3_out进行下采样，下采样后与P4_td、P4_in_2堆叠获得P4_out；之后对P4_out进行下采样，下采样后与P5_td、P5_in_2进行堆叠获得P5_out；之后对P5_out进行下采样，下采样后与P6_in、P6_td进行堆叠获得P6_out；之后对P6_out进行下采样，下采样后与P7_in进行堆叠获得P7_out。
实现代码如下：
#--------------------------构建BIFPN的上下采样循环-------------------------#
P3_D = layers.MaxPooling2D(pool_size=3, strides=2, padding='same')(P3_out)
P4_out = wBiFPNAdd(name=f'fpn_cells/cell_{id}/fnode4/add')([P4_in_2, P4_td, P3_D])
P4_out = layers.Activation(lambda x: tf.nn.swish(x))(P4_out)
P4_out = SeparableConvBlock(num_channels=num_channels, kernel_size=3, strides=1,
name=f'fpn_cells/cell_{id}/fnode4/op_after_combine9')(P4_out)
P4_D = layers.MaxPooling2D(pool_size=3, strides=2, padding='same')(P4_out)
P5_out = wBiFPNAdd(name=f'fpn_cells/cell_{id}/fnode5/add')([P5_in_2, P5_td, P4_D])
P5_out = layers.Activation(lambda x: tf.nn.swish(x))(P5_out)
P5_out = SeparableConvBlock(num_channels=num_channels, kernel_size=3, strides=1,
name=f'fpn_cells/cell_{id}/fnode5/op_after_combine10')(P5_out)
P5_D = layers.MaxPooling2D(pool_size=3, strides=2, padding='same')(P5_out)
P6_out = wBiFPNAdd(name=f'fpn_cells/cell_{id}/fnode6/add')([P6_in, P6_td, P5_D])
P6_out = layers.Activation(lambda x: tf.nn.swish(x))(P6_out)
P6_out = SeparableConvBlock(num_channels=num_channels, kernel_size=3, strides=1,
name=f'fpn_cells/cell_{id}/fnode6/op_after_combine11')(P6_out)
P6_D = layers.MaxPooling2D(pool_size=3, strides=2, padding='same')(P6_out)
P7_out = wBiFPNAdd(name=f'fpn_cells/cell_{id}/fnode7/add')([P7_in, P6_D])
P7_out = layers.Activation(lambda x: tf.nn.swish(x))(P7_out)
P7_out = SeparableConvBlock(num_channels=num_channels, kernel_size=3, strides=1,
name=f'fpn_cells/cell_{id}/fnode7/op_after_combine12')(P7_out)
#-------------------------------------------------------------------------#
4、将获得的P3_out、P4_out、P5_out、P6_out、P7_out作为P3_in、P4_in、P5_in、P6_in、P7_in，重复2、3步骤进行堆叠即可，对于Effiicientdet B0来讲，还需要重复2次，需要注意P4_in_1和P4_in_2此时不需要分开了，P5也是。
实现代码如下：
P3_in, P4_in, P5_in, P6_in, P7_in = features
P7_U = layers.UpSampling2D()(P7_in)
P6_td = wBiFPNAdd(name=f'fpn_cells/cell_{id}/fnode0/add')([P6_in, P7_U])
P6_td = layers.Activation(lambda x: tf.nn.swish(x))(P6_td)
P6_td = SeparableConvBlock(num_channels=num_channels, kernel_size=3, strides=1,
name=f'fpn_cells/cell_{id}/fnode0/op_after_combine5')(P6_td)
P6_U = layers.UpSampling2D()(P6_td)
P5_td = wBiFPNAdd(name=f'fpn_cells/cell_{id}/fnode1/add')([P5_in, P6_U])
P5_td = layers.Activation(lambda x: tf.nn.swish(x))(P5_td)
P5_td = SeparableConvBlock(num_channels=num_channels, kernel_size=3, strides=1,
name=f'fpn_cells/cell_{id}/fnode1/op_after_combine6')(P5_td)
P5_U = layers.UpSampling2D()(P5_td)
P4_td = wBiFPNAdd(name=f'fpn_cells/cell_{id}/fnode2/add')([P4_in, P5_U])
P4_td = layers.Activation(lambda x: tf.nn.swish(x))(P4_td)
P4_td = SeparableConvBlock(num_channels=num_channels, kernel_size=3, strides=1,
name=f'fpn_cells/cell_{id}/fnode2/op_after_combine7')(P4_td)
P4_U = layers.UpSampling2D()(P4_td)
P3_out = wBiFPNAdd(name=f'fpn_cells/cell_{id}/fnode3/add')([P3_in, P4_U])
P3_out = layers.Activation(lambda x: tf.nn.swish(x))(P3_out)
P3_out = SeparableConvBlock(num_channels=num_channels, kernel_size=3, strides=1,
name=f'fpn_cells/cell_{id}/fnode3/op_after_combine8')(P3_out)
P3_D = layers.MaxPooling2D(pool_size=3, strides=2, padding='same')(P3_out)
P4_out = wBiFPNAdd(name=f'fpn_cells/cell_{id}/fnode4/add')([P4_in, P4_td, P3_D])
P4_out = layers.Activation(lambda x: tf.nn.swish(x))(P4_out)
P4_out = SeparableConvBlock(num_channels=num_channels, kernel_size=3, strides=1,
name=f'fpn_cells/cell_{id}/fnode4/op_after_combine9')(P4_out)
P4_D = layers.MaxPooling2D(pool_size=3, strides=2, padding='same')(P4_out)
P5_out = wBiFPNAdd(name=f'fpn_cells/cell_{id}/fnode5/add')([P5_in, P5_td, P4_D])
P5_out = layers.Activation(lambda x: tf.nn.swish(x))(P5_out)
P5_out = SeparableConvBlock(num_channels=num_channels, kernel_size=3, strides=1,
name=f'fpn_cells/cell_{id}/fnode5/op_after_combine10')(P5_out)
P5_D = layers.MaxPooling2D(pool_size=3, strides=2, padding='same')(P5_out)
P6_out = wBiFPNAdd(name=f'fpn_cells/cell_{id}/fnode6/add')([P6_in, P6_td, P5_D])
P6_out = layers.Activation(lambda x: tf.nn.swish(x))(P6_out)
P6_out = SeparableConvBlock(num_channels=num_channels, kernel_size=3, strides=1,
name=f'fpn_cells/cell_{id}/fnode6/op_after_combine11')(P6_out)
P6_D = layers.MaxPooling2D(pool_size=3, strides=2, padding='same')(P6_out)
P7_out = wBiFPNAdd(name=f'fpn_cells/cell_{id}/fnode7/add')([P7_in, P6_D])
P7_out = layers.Activation(lambda x: tf.nn.swish(x))(P7_out)
P7_out = SeparableConvBlock(num_channels=num_channels, kernel_size=3, strides=1,
name=f'fpn_cells/cell_{id}/fnode7/op_after_combine12')(P7_out)
