Add ResNet50 backbone

[rwightman]

As per tensorflow/tfjs-models#199

[ggzzzzz628]

How does the weight of tensorflow.js translate and adapt to this code? thank you very much!
@rwightman

[1220deeplearner]

How can I train my own model on COCO dataset?
Is it possible in PyTorch implementation?

[rwightman]

@1220deeplearner See #8