Fine-Tune part of Model

[ZahraAnam]

Dear Community,
I have a question has anyone fine-tuned only a part of model? I want to fine-tune faster-rcnn-X101 for custome dataset but I want to opt for the 4-Step Alternating Training strategy as per faster-rcnn paper but don't understand how to fine-tune just some parts of model.

If someone has done this or has any ideas, will be great help.