Compatibility with plaidml

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I was wondering how difficult it would be to adjust your code to run on the plaidml backend (https://github.com/plaidml/plaidml), which supports AMD GPUs on Mac. plaidml is currently only compatible with Keras 2.0.8, and it seems that some of your code would need to be adapted. There are also a few lines of code that use the tf backend and not the keras backend in the model.py file, i.e. in the yolo_eval and yolo_loss functions. Compared to other implementations of yolo in keras, your code seems to be much closer to be compatible with plaidml.