how to fine tune faster-rcnn-resnet-101-coco model with tensorflow object detection API ?

Clash Royale CLAN TAG#URR8PPPhow to fine tune faster-rcnn-resnet-101-coco model with tensorflow object detection API ?
I want to keep several layers in the faster rcnn resnet 101 model.
And change or train the last 2 layers so I can fine it tune to my own object detection problem which have less categories. I make much efforts but failed.
Firstly, the model file which the API saved did not keep the loss tensors.
Secondly, there were many branches in the tensorflow op so that I can't make it clear. Maybe I should modify the source code of the API but I failed. I was lost in it. PLZ help me.
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