How to fuse layers of pretrained model?

I am using the DeepLabV3 with MobileNetV3Large backbone. What could be the good way to fuse the layers ? All layer fusing examples I see is mostly for user defined models.
Thanks in avdance.

My model is as follows:

from torchvision import models
model = models.segmentation.deeplabv3_mobilenet_v3_large(weights=‘COCO_WITH_VOC_LABELS_V1’,progress=True)