This implements the paper:
Tsung-Yi Lin, Piotr Dollar, Ross Girshick, Kaiming He, Bharath Hariharan, and
Serge Belongie.
Feature Pyramid Networks for Object Detection.
(https://arxiv.org/pdf/1612.03144)
Args
input_specs
A dict of input specifications. A dictionary consists of
{level: TensorShape} from a backbone.
min_level
An int of minimum level in FPN output feature maps.
max_level
An int of maximum level in FPN output feature maps.
num_filters
An int number of filters in FPN layers.
fusion_type
A str of sum or concat. Whether performing sum or
concat for feature fusion.
use_separable_conv
A bool. If True use separable convolution for
convolution in FPN layers.
use_keras_layer
A bool. If Ture use keras layers as many as possible.
activation
A str name of the activation function.
use_sync_bn
A bool. If True, use synchronized batch normalization.
norm_momentum
A float of normalization momentum for the moving average.
norm_epsilon
A float added to variance to avoid dividing by zero.
kernel_initializer
A str name of kernel_initializer for convolutional
layers.