The functions are same as mentioned in the tutorials on the PyTorch website:

In references/detection/, we have a number of helper functions to simplify training and evaluating detection models. Here, we will use  references/detection/engine.py ,  references/detection/utils.py  and  references/detection/transforms.py . Just copy them to your folder and use them here.`

I got both my train and evaluate functions from here :  https://github.com/pytorch/vision/tree/master/references/detection 8